xutils: Rewrite pixmap + mak icon reading code to use cairo
xutils: Rewrite pixmap + mak icon reading code to use cairo

The previous code was hard-coded to assume certain alpha values. And
those values weren't always correct. In particular, the new GTK3 code
violated the assumptions.
The new code can cope with any alpha combination.

lanoxx/libwnck,lanoxx/libwnck