Shaders: have a definition for the generic color attribute.

src/Magnum/Shaders/Generic.h

@@ -45,6 +45,8 @@ See @ref shaders-generic for more information.
 template<UnsignedInt> struct Generic;
 #else
 template<UnsignedInt dimensions> struct Generic {
+    /* Keep consistent with generic.glsl and the real definitions below */
+
     /**
      * @brief Vertex position
      *

src/Magnum/Shaders/VertexColor2D.vert

@@ -43,7 +43,7 @@ layout(location = POSITION_ATTRIBUTE_LOCATION)
 in highp vec2 position;
 
 #ifdef EXPLICIT_ATTRIB_LOCATION
-layout(location = 3)
+layout(location = COLOR_ATTRIBUTE_LOCATION)
 #endif
 in lowp vec4 color;
 

src/Magnum/Shaders/VertexColor3D.vert

@@ -43,7 +43,7 @@ layout(location = POSITION_ATTRIBUTE_LOCATION)
 in highp vec4 position;
 
 #ifdef EXPLICIT_ATTRIB_LOCATION
-layout(location = 3)
+layout(location = COLOR_ATTRIBUTE_LOCATION)
 #endif
 in lowp vec4 color;
 

src/Magnum/Shaders/generic.glsl

@@ -23,6 +23,9 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+/* Keep consistent with Generic.h */
+
 #define POSITION_ATTRIBUTE_LOCATION 0
 #define TEXTURECOORDINATES_ATTRIBUTE_LOCATION 1
 #define NORMAL_ATTRIBUTE_LOCATION 2
+#define COLOR_ATTRIBUTE_LOCATION 3