Shaders: assume materialId is 0 if MATERIAL_COUNT is 1.

Unlike the drawId optimization before, there's no possibility to
check this anywhere, so the assumption is just documented.

On an Intel 630 this resulted in further significant reduction for the
single-draw single-material case, down to 260 from 440 in the previous
commit, about a 45% reduction compared to the original 550 ms; multidraw
case is still around the 550 there.

src/Magnum/Shaders/DistanceFieldVector.frag

@@ -144,7 +144,11 @@ out lowp vec4 fragmentColor;
 
 void main() {
     #ifdef UNIFORM_BUFFERS
+    #if MATERIAL_COUNT > 1
     mediump const uint materialId = draws[drawId].draw_materialIdReserved & 0xffffu;
+    #else
+    #define materialId 0u
+    #endif
     lowp const float smoothness = materials[materialId].material_smoothness;
     lowp const vec4 color = materials[materialId].color;
     lowp const vec4 outlineColor = materials[materialId].outlineColor;

src/Magnum/Shaders/DistanceFieldVector.h

@@ -101,8 +101,9 @@ struct DistanceFieldVectorDrawUniform {
      * more than one material is supplied or in a multi-draw scenario. Should
      * be less than the material count passed to the
      * @ref DistanceFieldVectorGL::DistanceFieldVectorGL(Flags, UnsignedInt, UnsignedInt)
-     * constructor. Default value is @cpp 0 @ce, meaning the first material
+     * constructor, if material count is @cpp 1 @ce, this field is assumed to
-     * gets used.
+     * be @cpp 0 @ce and isn't even read by the shader. Default value is
+     * @cpp 0 @ce, meaning the first material gets used.
      */
 
     /* This field is an UnsignedInt in the shader and materialId is extracted

src/Magnum/Shaders/Flat.frag

@@ -176,7 +176,11 @@ void main() {
     #ifdef OBJECT_ID
     highp const uint objectId = draws[drawId].draw_objectId;
     #endif
+    #if MATERIAL_COUNT > 1
     mediump const uint materialId = draws[drawId].draw_materialIdReserved & 0xffffu;
+    #else
+    #define materialId 0u
+    #endif
     lowp const vec4 color = materials[materialId].color;
     #ifdef ALPHA_MASK
     lowp const float alphaMask = materials[materialId].material_alphaMask;

src/Magnum/Shaders/Flat.h

@@ -108,8 +108,10 @@ struct FlatDrawUniform {
      * References a particular material from a @ref FlatMaterialUniform array.
      * Useful when an UBO with more than one material is supplied or in a
      * multi-draw scenario. Should be less than the material count passed to
-     * the @ref FlatGL::FlatGL(Flags, UnsignedInt, UnsignedInt) constructor.
+     * the @ref FlatGL::FlatGL(Flags, UnsignedInt, UnsignedInt) constructor, if
-     * Default value is @cpp 0 @ce, meaning the first material gets used.
+     * material count is @cpp 1 @ce, this field is assumed to be @cpp 0 @ce and
+     * isn't even read by the shader. Default value is @cpp 0 @ce, meaning the
+     * first material gets used.
      */
 
     /* This field is an UnsignedInt in the shader and materialId is extracted

src/Magnum/Shaders/MeshVisualizer.frag

@@ -224,7 +224,11 @@ out lowp vec4 fragmentColor;
 
 void main() {
     #ifdef UNIFORM_BUFFERS
+    #if MATERIAL_COUNT > 1
     mediump const uint materialId = draws[drawId].draw_materialIdReserved & 0xffffu;
+    #else
+    #define materialId 0u
+    #endif
     #if (defined(WIREFRAME_RENDERING) || defined(INSTANCED_OBJECT_ID) || defined(VERTEX_ID) || defined(PRIMITIVE_ID) || defined(PRIMITIVE_ID_FROM_VERTEX_ID)) && !defined(TBN_DIRECTION)
     lowp const vec4 color = materials[materialId].color;
     lowp const vec4 wireframeColor = materials[materialId].wireframeColor;

src/Magnum/Shaders/MeshVisualizer.h

@@ -100,8 +100,9 @@ struct MeshVisualizerDrawUniform2D {
      * more than one material is supplied or in a multi-draw scenario. Should
      * be less than the material count passed to the @ref MeshVisualizerGL2D::MeshVisualizerGL2D(Flags, UnsignedInt, UnsignedInt)
      * / @ref MeshVisualizerGL3D::MeshVisualizerGL3D(Flags, UnsignedInt, UnsignedInt)
-     * constructor. Default value is @cpp 0 @ce, meaning the first material
+     * constructor, if material count is @cpp 1 @ce, this field is assumed to
-     * gets used.
+     * be @cpp 0 @ce and isn't even read by the shader. Default value is
+     * @cpp 0 @ce, meaning the first material gets used.
      */
 
     /* This field is an UnsignedInt in the shader and materialId is extracted

src/Magnum/Shaders/Phong.frag

@@ -339,7 +339,11 @@ void main() {
     #ifdef OBJECT_ID
     highp const uint objectId = draws[drawId].draw_objectId;
     #endif
+    #if MATERIAL_COUNT > 1
     mediump const uint materialId = draws[drawId].draw_materialIdReserved & 0xffffu;
+    #else
+    #define materialId 0u
+    #endif
     lowp const vec4 ambientColor = materials[materialId].ambientColor;
     #if LIGHT_COUNT
     lowp const vec4 diffuseColor = materials[materialId].diffuseColor;

src/Magnum/Shaders/Phong.h

@@ -141,8 +141,9 @@ struct PhongDrawUniform {
      * Useful when a UBO with more than one material is supplied or in a
      * multi-draw scenario. Should be less than the material count passed to
      * the @ref PhongGL::PhongGL(Flags, UnsignedInt, UnsignedInt, UnsignedInt)
-     * constructor. Default value is @cpp 0 @ce, meaning the first material
+     * constructor, if material count is @cpp 1 @ce, this field is assumed to
-     * gets used.
+     * be @cpp 0 @ce and isn't even read by the shader. Default value is
+     * @cpp 0 @ce, meaning the first material gets used.
      */
 
     /* This field is an UnsignedInt in the shader and materialId is extracted

src/Magnum/Shaders/Vector.frag

@@ -123,7 +123,11 @@ out lowp vec4 fragmentColor;
 
 void main() {
     #ifdef UNIFORM_BUFFERS
+    #if MATERIAL_COUNT > 1
     mediump const uint materialId = draws[drawId].draw_materialIdReserved & 0xffffu;
+    #else
+    #define materialId 0u
+    #endif
     lowp const vec4 color = materials[materialId].color;
     lowp const vec4 backgroundColor = materials[materialId].backgroundColor;
     #endif

src/Magnum/Shaders/Vector.h

@@ -96,8 +96,9 @@ struct VectorDrawUniform {
      * array. Useful when an UBO with more than one material is supplied or in
      * a multi-draw scenario. Should be less than the material count passed to
      * the @ref VectorGL::VectorGL(Flags, UnsignedInt, UnsignedInt)
-     * constructor. Default value is @cpp 0 @ce, meaning the first material
+     * constructor, if material count is @cpp 1 @ce, this field is assumed to
-     * gets used.
+     * be @cpp 0 @ce and isn't even read by the shader. Default value is
+     * @cpp 0 @ce, meaning the first material gets used.
      */
 
     /* This field is an UnsignedInt in the shader and materialId is extracted