gl-renderer: Clean up egl attach failure path

Try to make the error handling code a little clearer. Instead of always setting num_image to the expected number of images and having a sparsely correct array of images when we fail, stop on the first failure and do some immediate clean up. Then re-use the censor placeholder (solid brown) for incomplete surface.

This leads to very visibly ugly failures - since we only update damaged regions, pieces of the placeholder color may remain for a long time.

Arguably, trying to make failures look good isn't the compositor's responsibility, and if people report "there are ugly black spots in my windows" we'll be able to diagnose client failure easily.

Removes the comment in egl_image_unref - but we still allow unreffing a NULL image without error, as those are sensible semantics.

Signed-off-by: Derek Foreman derek.foreman@collabora.com