There is a patch available for media-video/mplayer-1.0_rc2 that would allow for potential performance gains when using compiz(-fusion) by using compiz's own OpenGL functions to draw the video. Patch is attached.
Created attachment 133804 [details, diff] mplayer-compiz-opengl.patch
Please, submit such things upstream, this is not a proper place for feature requests. http://www.mplayerhq.hu/design7/mailing_lists.html
This was already mostly rejected in the current form due to the ugly way it is mangled into -vo xv. There was also no argument given why -vo gl wouldn't work just as well (or at least what the specific differences are).
Reimar, thanks for information. Closing this as we are not going to apply anything that's rejected by upstream unless it's absolutely mandatory.
*** Bug 203244 has been marked as a duplicate of this bug. ***