uterm: video: add uterm_display_get_buffer()

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>