Clear alpha in xRGB destination to avoid pixman setting it to 0xff
Clear alpha in xRGB destination to avoid pixman setting it to 0xff

Pixman sometimes sets the ignored high byte to 0xff during alpha
blending. This is correct according to pixman specs, as the high
byte is ignored. However its not what windows expects, and it causes
unnecessary regions with non-zero high byte, causing us to
send rgba data instead of rgb which compresses worse.

So, we detect this and clear the high byte.

Fantu/spice-common,fgouget/spice-common,SPICE/spice-common,elmarco/spice-common,Fantu/spice-common,freedesktop-unofficial-mirror/spice__spice-common,elmarco/spice-common,fgouget/spice-common,elmarco/spice-common,fgouget/spice-common,elmarco/spice-common,freedesktop-unofficial-mirror/spice__spice-common,SPICE/spice-common,freedesktop-unofficial-mirror/spice__spice-common,SPICE/spice-common,Fantu/spice-common,fgouget/spice-common,freedesktop-unofficial-mirror/spice__spice-common,Fantu/spice-common