cogl/framebuffer: Remove requirement to discard color buffer

cogl_framebuffer_discard_buffers required that the color buffer is
passed, although users might want to discard e.g. just depth and/or
stencil buffers.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2091>

(cherry picked from commit 5fedd065)