Add overlay code and simplify access to @gl-context

Previously we were willy-nilly just accessing @gl-context. Part of this
commit aims to reduce synced actions in the main code path.

Some regressions with respect to reflection have been introduced in this
commit. They will be dealt with at a different time.
Panel.cfg reader works! Showing a simple panel.

Still need to adjust view offsets and handle color keyed transparency,
but overall looking good