draw: Add draw_set_index_buffer and others.

draw: Add draw_set_index_buffer and others.

This commit adds draw_set_index_buffer, draw_set_mapped_index_buffer,
and draw_vbo.  The idea behind the new functions is that an index buffer
should be a state.

draw_arrays and draw_set_mapped_element_buffer are preserved, but the
latter will be removed soon.