Warn if pixman_blt fails in screen share

Currently if pixman_blt fails we silently ignore this. Typically pixman_blt fails if there is no accelerated version of it available. It seems that pixman_blt expects the user to fall back to a local implementation in case pixman has no accelerated version available. Warn in this case so the user has at least a hint what went wrong.

References #253 (closed)