Slight code optimization for CopyArea and CopyWindow in rpi_x.c
Properly use accelerated CPU back-end with RPI AccelMethod

When the "AccelMethod" "RPI" is specified, the accelerated CPU back-end
is now actually used. It was not before.

It is not recommended to use the "AccelMethod" "RPI" method currently
because no hardware acceleration functions are implemented yet. It only
add some overhead.