glcolorconvert: fix wrong RGB to YUV matrix with bt709
requested to merge vengzor/gstreamer:glcolorconvert-fix-wrong-rgb-yuv-colormatrix-with-bt709 into main
when converting from RGB to YUV: Comparing the info.colorimetry to GST_VIDEO_COLORIMETRY_BT709 it does not make sense to look at the input "signal" as this is some type of RGB. The plugin needs to look at the output and compare GST_VIDEO_COLORIMETRY_BT709 to that, because that is the YUV-type the plugin needs to convert it RGB into. when converting from YUV to RGB it is correct to look at the input, but because here the color encoding info BT601/BT709 is on that "side" of the plugin.
I made an issue at some point here. To be closed if changes are accepted #2948 (comment 2062995)