These are necessary if nesting redirections to an fbo,
otherwise there's no way to know how to restore
previous state.
glPushAttrib(GL_COLOR_BUFFER_BIT) would save draw buffer
state, but also saves a lot of other stuff, and
cogl_draw_buffer() relies on knowing about all draw
buffer state changes. So we have to implement a
draw buffer stack ourselves.
Signed-off-by: Robert Bragg <robert@linux.intel.com>
e47b198225