gl: ignore if mask has not bitmap
gl: ignore if mask has not bitmap

Similar to how sw canvas ignore mask if !bitmap in canvas_mask_pixman().

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