textoverlay: Shading behaves differently with RGB and YUV formats
Compare:
$ gst-launch-1.0 videotestsrc ! video/x-raw,width=1920,height=1080 ! clockoverlay shaded-background=true shading-value=255 draw-outline=false draw-shadow=false valignment=bottom halignment=center ! video/x-raw,format=I420 ! videoconvert ! autovideosink
$ gst-launch-1.0 videotestsrc ! video/x-raw,width=1920,height=1080 ! clockoverlay shaded-background=true shading-value=255 draw-outline=false draw-shadow=false valignment=bottom halignment=center ! video/x-raw,format=xRGB ! videoconvert ! autovideosink
I don't know which behaviour is intended but it should probably behave the same independent of the format.