compositor: Use 255 as maximum alpha instead of 256

compositor: Use 255 as maximum alpha instead of 256

255 will easily become 0 in the blending function as they expect
the maximum value to be 255.

Can be reproduce with

gst-launch-1.0 videotestsrc pattern=ball ! c.sink_0 \
               videotestsrc pattern=snow ! c.sink_1 \
               compositor name=c \
                 sink_0::zorder=0 sink_1::zorder=1 sink_0::crossfade-ratio=0.5 \
                 background=black ! \
               videoconvert ! xvimagesink

crossfade-ratio +/- 0.001 makes it work correctly and the same happens
at e.g. 0.25, 0.75, N*0.0625

https://bugzilla.gnome.org/show_bug.cgi?id=796846