Backport of "qtmux: for Apple ProRes, allow overriding pixel bit depth, e.g. when exporting an opaque image, yet with alpha." into 1.18
requested to merge tpm/gst-plugins-good:backport-mr-1061-into-1.18-ruslank-master-patch-11248 into 1.18
Backport of !1061 (merged) "qtmux: for Apple ProRes, allow overriding pixel bit depth, e.g. when exporting an opaque image, yet with alpha.".
Commits: