MeshTools: return pointer to buffer from fullScreenTriangle().

On systems without VAOs and without gl_VertexID the buffer needs to be
bound before every draw call. This way the buffer reference would become
dangling one, because the buffer is moved out somewhere else. Now
creating it on heap and returning pointer instead.

This multiple code path implementation is really starting to pay out,
yay!
Merge branch 'master' into compatibility