Assorted fixes for dealing with zero-size frame/renderbuffers.

Assorted fixes for dealing with zero-size frame/renderbuffers.

In xmesa_check_and_update_buffer_size() handle xmctx==NULL correctly: still
call _mesa_resize_framebufer().  If we don't we can wind up in a situation
where the framebuffer size is non-zero but an attached renderbuffer size
is still initialized to zero.  This inconsistancy can later cause problems.
Check for zero-size renderbuffers in update_color_draw_buffers() and
update_color_read_buffer().
See bug 7205.