The current mythtv ebuild (0.19_p10281) isn't compatible with the switch from nvidia-kernel and nvidia-glx to nvidia-drivers - it still DEPENDs on nvidia-glx (if you have nvidia in your VIDEO_CARDS and xvmc in your USE). Attached is an updated ebuild that corrects this.
Created attachment 91158 [details] mythtv-0.19_p10281.ebuild Corrected mythtv-0.19_p10281.ebuild
Fixed.