"Recently, the openchrome project renamed the VIA video drivers that they maintain. As a result libviaXvMC became libchromeXvMC and libviaXvMCPro became libchromeXvMCPro. This has caused MythTV's configure script to fail to detect the libraries, which results in XvMC-VLD being disabled." Applying the patch from the above bug at the end of src_unpack() allows ./configure to detect the openchrome libraries and not override your use of --enable-xvmc-pro.
Created attachment 143958 [details, diff] Patch to fix the openchrome detection
As per upstream, we do not patch MythTV in anyway. Especially not the 0.21_pre series which are merely trunk snapshots. This patch would be better suited on their bug tracker, http://svn.mythtv.org Ensure you go to Preferences first and save your name and e-mail address due to a bug in their bug tracker.
I didn't notice that you had actually submitted it upstream. Either way, this will be in the forth coming beta ebuilds.
The upstream patch is not mine, I found it when looking for a solution to missing XvMC and though it would be worth adding to the ebuild.