Shaders: set MeshVisualizer::VertexIndex attribute location to 3.

Locations 0, 1 and 2 will be reserved for positions, texture coordinates
and normals.

src/Shaders/MeshVisualizer.h

@@ -77,7 +77,7 @@ class MAGNUM_SHADERS_EXPORT MeshVisualizer: public AbstractShaderProgram {
          * OpenGL 3.1, OpenGL ES 3.0 and newer this value is provided by the
          * shader itself, so the attribute is not needed.
          */
-        typedef Attribute<1, Float> VertexIndex;
+        typedef Attribute<3, Float> VertexIndex;
 
         /**
          * @brief %Flag

src/Shaders/MeshVisualizer.vert

@@ -42,7 +42,7 @@ 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 = 1) in lowp float vertexIndex;
+layout(location = 3) in lowp float vertexIndex;
 #else
 in lowp float vertexIndex;
 #endif