Handle 8 and 10 bits colour depth in picking code (#11)

This is a potential resolution for issue #11 (closed)

Note that I tested this merge request with 10 and 8 bpc (allow_rgb10_configs=false) under Wayland, however it should be reviewed by someone more experienced by myself.

One particularity is that I would expect the pixel format passed to cogl_read_pixels to be COGL_PIXEL_FORMAT_RGBA_1010102_PRE for 10 bpc, but it appears that this isn't the case. COGL_PIXEL_FORMAT_RGBA_8888_PRE works fine for 8/10bpc on my system. Testing recommended with the actor-pick test under tests/conform.