mjpeg_encoder: use libjpeg-turbo extra colorspaces
mjpeg_encoder: use libjpeg-turbo extra colorspaces

When libjpeg-turbo is available, we can use the BGR and BGRX
colorspaces that it provides to avoid extra conversions of the
data we want to compress to mjpeg

dezelin/spice,Fantu/spice,ya-mouse/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,mathslinux/spice,fgouget/spice,mathslinux/spice,elmarco/spice,elmarco/spice,elmarco/spice,fgouget/spice,SPICE/spice,elmarco/spice,mathslinux/spice,dezelin/spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,ya-mouse/spice,freedesktop-unofficial-mirror/spice__spice,mathslinux/spice,dezelin/spice,dezelin/spice,alon/spice-server,freedesktop-unofficial-mirror/spice__spice,mathslinux/spice,alon/spice-server,elmarco/spice,alon/spice-server,fgouget/spice,Fantu/spice,ya-mouse/spice,Fantu/spice,Fantu/spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,SPICE/spice,dezelin/spice,SPICE/spice