Shaders: avoid MeshVisualizer::VertexIndex reusing a generic color slot.

Can't really future-proof that, so just adding a lot of comments
everywhere.

src/Magnum/Shaders/Generic.h

@@ -66,6 +66,10 @@ template<UnsignedInt dimensions> struct Generic {
         #endif
     };
 
+    /* Index 5 used by MeshVisualizer::VertexIndex. Update
+       MeshVisualizerTest::vertexIndexNoConflict() when adding new attributes
+       here. */
+
     /**
      * @brief Vertex position
      *

src/Magnum/Shaders/MeshVisualizer.h

@@ -125,7 +125,7 @@ class MAGNUM_SHADERS_EXPORT MeshVisualizer: public GL::AbstractShaderProgram {
          * @cb{.glsl} gl_VertexID @ce shader builtin, so the attribute is not
          * needed.
          */
-        typedef GL::Attribute<3, Float> VertexIndex;
+        typedef GL::Attribute<5, Float> VertexIndex;
 
         enum: UnsignedInt {
             /**

src/Magnum/Shaders/MeshVisualizer.vert

@@ -45,7 +45,7 @@ in highp vec4 position;
 #if defined(WIREFRAME_RENDERING) && defined(NO_GEOMETRY_SHADER)
 #if (!defined(GL_ES) && __VERSION__ < 140) || (defined(GL_ES) && __VERSION__ < 300)
 #ifdef EXPLICIT_ATTRIB_LOCATION
-layout(location = 3)
+layout(location = 5)
 #endif
 in lowp float vertexIndex;
 #define gl_VertexID int(vertexIndex)

src/Magnum/Shaders/Test/MeshVisualizerTest.cpp

@@ -37,6 +37,8 @@ struct MeshVisualizerTest: TestSuite::Tester {
     void constructNoCreate();
     void constructCopy();
 
+    void vertexIndexNoConflict();
+
     void debugFlag();
     void debugFlags();
 };
@@ -45,6 +47,8 @@ MeshVisualizerTest::MeshVisualizerTest() {
     addTests({&MeshVisualizerTest::constructNoCreate,
               &MeshVisualizerTest::constructCopy,
 
+              &MeshVisualizerTest::vertexIndexNoConflict,
+
               &MeshVisualizerTest::debugFlag,
               &MeshVisualizerTest::debugFlags});
 }
@@ -63,6 +67,15 @@ void MeshVisualizerTest::constructCopy() {
     CORRADE_VERIFY(!(std::is_assignable<MeshVisualizer, const MeshVisualizer&>{}));
 }
 
+void MeshVisualizerTest::vertexIndexNoConflict() {
+    CORRADE_VERIFY(MeshVisualizer::VertexIndex::Location != Generic3D::Position::Location);
+    CORRADE_VERIFY(MeshVisualizer::VertexIndex::Location != Generic3D::Normal::Location);
+    CORRADE_VERIFY(MeshVisualizer::VertexIndex::Location != Generic3D::TextureCoordinates::Location);
+    CORRADE_VERIFY(MeshVisualizer::VertexIndex::Location != Generic3D::Color3::Location);
+    CORRADE_VERIFY(MeshVisualizer::VertexIndex::Location != Generic3D::Color4::Location);
+    CORRADE_VERIFY(MeshVisualizer::VertexIndex::Location != Generic3D::Tangent::Location);
+}
+
 void MeshVisualizerTest::debugFlag() {
     std::ostringstream out;