uterm: video: add uterm_display_get_buffer()

Until now we required external renderers to use the blit/blend helpers.
However, this seems hardly reasonable now so we provide a way to retrieve
the 2D buffer via the public API.

This is limited to xrgb32 and rgb16, other formats are not exported for
now (and probably not used, yet?). The 3D backends do not support that so
use the blit/blend helpers or OpenGL.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
shl: hook: fix invalid memory access for non-oneshot entries

If an entry is not a oneshot entry, then it might get deleted during the
callback procedure and hence, we _must_ not access it afterwards.
Therefore, remember the oneshot setting so we access it only if it really
is a oneshot entry.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>