VAAPI hwdec

[yuv418]

• The basic backend works
• Fixing random bugs (A)
  • Sometimes the backend loses a SPS/PPS and dies (this seems weird since I believe NVENC at least will resend SPSPPS).
  • Sometimes the backend has an issue where it says it "subtracts with overflow"
• Performance analysis to reduce latency (right now, the latency looks to be worse than OpenH264, perhaps there is needless copying). Maybe add a couple of statistics for VA-API specific things.
• Investigate difference between Blocking and NonBlocking modes and figure out why NonBlocking seems to have the random bugs referenced in (A) more often. Evaluate the performance of the modes/if it makes a difference