Move Spritesheet onto VertexBuffer.

We now use G_ARB_draw_elements_base_vertex extension, so that
instead of having to create tons of index buffers (one per tile),
we just use the default one {0, 1, 3, 2}, then to use the correct
vertices in the VBO for that tile, we use the base_vertex parameter
of the glDrawElementsBaseVertex, so that the indices are offset to
n + base_vertex.