compositor: improve opacity handling for scaled surfaces

Currently, the opaque is discarded for all transformations other than a simple
translation, because correctly transforming the opaque area is not possible in
general.
However, there is one simple case that is probably the most common one: A fully
opaque surface that is translated and scaled. In this case the opaque area is
simply the new bounding box. So set the transformed opaque area accordingly.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>