So I spent the day writing Xv support for glamor for no good reason,
contains the result of my day, the glamor repo may not be public yet, its waiting on fd.o cgit crawler.
Xv works for YV12 planar videos, I suspect to do packed video support I'd need a GL extension to expose the hw formats for doing packed video, this probably wouldn't be a major extension and maybe someone might do it sometime.
The code supports, brightness, contrast, hue and saturation controls using the code ported from the radeon driver.
I've tested it with mplayer on evergreen card of some variant, and it seems to work fine with the one video I used :-)