Allow attribute-less Mesh.

For e.g. fullscreen triangle with no vertex data.

src/Mesh.cpp

@@ -111,7 +111,7 @@ void Mesh::bindVAO(GLuint vao) {
 }
 
 void Mesh::bind() {
-    CORRADE_ASSERT((_vertexCount == 0) == attributes.empty(), "Mesh: vertex count is non-zero, but no attributes are bound", );
+    CORRADE_ASSERT((_vertexCount != 0) || !attributes.empty(), "Mesh: attributes are bound but vertex count is zero", );
 
     (this->*bindImplementation)();
 }

src/Mesh.h

@@ -404,8 +404,9 @@ class MAGNUM_EXPORT Mesh {
          *
          * Default is zero.
          * @attention All bound attributes are reset after calling this
-         *      function, so you must call
+         *      function, so if your mesh has any vertex attributes, be sure
-         *      addVertexBuffer()/addInterleavedVertexBuffer() afterwards.
+         *      to call addVertexBuffer()/addInterleavedVertexBuffer()
+         *      afterwards.
          * @see MeshTools::interleave()
          */
         Mesh* setVertexCount(GLsizei vertexCount);