uterm: video: return buffer information on display_use()

uterm: video: return buffer information on display_use()

uterm_display_use() now returns the current back-buffer index when called.
It returns <0 on error. Whether OpenGL is supported is returned via a new
parameter "opengl". Set it to NULL if you're not interested.

Note that a backend might support OpenGL _and_ memory-mapped buffer
access. But you shouldn't rely on uterm_display_use() to return the
correct buffer-index if uterm_display_get_buffers() is not supported. For
instance the DRM-3D backend always returns 0 as buffer index as it has no
way of detecting it, yet.

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