nvdec: Implement VA-API hardware video acceleration (#6713)

* nvdec: VA-API

* Verify formatting

* Forgot a semicolon for Windows

* Clarify comment about AV_PIX_FMT_NV12

* Fix assert log spam from missing negation

* vic: Remove forgotten debug code

* Address lioncash's review

* Mention VA-API is Intel/AMD

* Address v1993's review

* Hopefully fix CMakeLists style this time

* vic: Improve cache locality

* vic: Fix off-by-one error

* codec: Async

* codec: Forgot the GetValue()

* nvdec: Address ameerj's review

* codec: Fallback to CPU without VA-API support

* cmake: Address lat9nq's review

* cmake: Make VA-API optional

* vaapi: Multiple GPU

* Apply suggestions from code review

Co-authored-by: Ameer J <52414509+ameerj@users.noreply.github.com>

* nvdec: Address ameerj's review

* codec: Use anonymous instead of static

* nvdec: Remove enum and fix memory leak

* nvdec: Address ameerj's review

* codec: Remove preparation for threading

Co-authored-by: Ameer J <52414509+ameerj@users.noreply.github.com>