Fix on-the-fly transparency changes in pixman-renderer
Fix on-the-fly transparency changes in pixman-renderer

This fixes :
- leaking the mask used to simulate transparency ;
- code style (definitions moved up, use of brackets) ;
- applying an opaque region when transparency is
 wanted (shound not happen).

Signed-off-by: Manuel Bachmann <d58f3f53462d5b02b85dbad407eeedd70c66c30c@open.eurogiciel.org>

Tarnyko/weston-xdg_surface_present,xorgy/weston,giucam/weston,Fantu/compositor-spice,sir-murray/weston,krezovic/weston,Gnurou/weston,jonnylamb/weston,Tarnyko/weston-xdg_surface_present,giucam/weston,sir-murray/weston,mchalupa/weston,mchalupa/weston,eyolfson/weston,Fantu/compositor-spice,krezovic/weston,xorgy/weston,udoprog/weston,kwm81/weston,eyolfson/weston,jonnylamb/weston,Gnurou/weston,Fantu/compositor-spice,kwm81/weston,udoprog/weston