st/mesa: flush the glBitmap cache before changing framebuffer state

st/mesa: flush the glBitmap cache before changing framebuffer state

This fixes the piglit EXT_framebuffer_multisample/bitmap tests.

Note that we must not rely on ctx->DrawBuffer when flushing the cache, because
that's already updated with a new framebuffer. We want to draw into the old
framebuffer where glBitmap was called.

Reviewed-by: Brian Paul <brianp@vmware.com>