VBO offscreen selection drawing, cdderivedmesh

Get rid of legacy drawing, it's only used for selection,
    in which case we can prepare a temporary color buffer and draw
    at once. Code is not complete here because we still redundantly
    set the draw color in the draw function and don't ommit hidden
    faces automatically. Still it works 100% without immediate mode
    now.

source/blender/gpu/GPU_buffers.h

@@ -172,6 +172,7 @@ void GPU_uv_setup(struct DerivedMesh *dm);
 void GPU_texpaint_uv_setup(struct DerivedMesh *dm);
 /* colType is the cddata MCol type to use! */
 void GPU_color_setup(struct DerivedMesh *dm, int colType);
+void GPU_buffer_bind_as_color(GPUBuffer *buffer);
 void GPU_edge_setup(struct DerivedMesh *dm); /* does not mix with other data */
 void GPU_uvedge_setup(struct DerivedMesh *dm);