media-video/mpv: add vulkan support to 9999

media-video/mpv: add vulkan support to 9999

Implementation details:
- mpv requires at least one enabled vulkan context (X or wayland),
  even though this isn't reflected by the build system. Hence the
  REQUIRED_USE. Without this you cannot enable vulkan backend.
- The appropriate context must be enabled in vulkan-loader.
- mpv technically allows vulkan without shaderc on NVIDIA cards,
  but support for this in NVIDIA drivers is deprecated and going
  away, see [1]. Thus always require shaderc.

[1]: https://github.com/mpv-player/mpv/issues/5867
Package-Manager: Portage-2.3.40, Repoman-2.3.9