widget: apply color conversion when creating image
widget: apply color conversion when creating image

The color conversion only happened during "invalidate", but we also
need to apply it when the image is created in the first place.

This solves initial screen being black after connection to agent-less
guest with 16b colour depth:

https://bugzilla.redhat.com/show_bug.cgi?id=843134

freedesktop-unofficial-mirror/spice__spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk,elmarco/spice-gtk,mathslinux/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,mathslinux/spice-gtk,SPICE/spice-gtk,mathslinux/spice-gtk,flexVDI/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,Fantu/spice-gtk,mathslinux/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk