lavc/vaapi_encode: enable 8bit 4:4:4 encoding for HEVC and VP9

Sufficiently recent Intel hardware is able to do encoding of 8bit 4:4:4
content in HEVC and VP9. The main requirement here is that the frames
must be provided in the AYUV format.

Enabling support is done by adding the appropriate encoding profiles
and noting that AYUV is officially a four channel format with alpha so
we must state that we expect all four channels.

libavcodec/vaapi_encode.c

@@ -1308,6 +1308,7 @@ static const VAAPIEncodeRTFormat vaapi_encode_rt_formats[] = {
     { "YUV422_10", VA_RT_FORMAT_YUV422_10,    10, 3, 1, 0 },
 #endif
     { "YUV444",    VA_RT_FORMAT_YUV444,        8, 3, 0, 0 },
+    { "AYUV",      VA_RT_FORMAT_YUV444,        8, 4, 0, 0 },
     { "YUV411",    VA_RT_FORMAT_YUV411,        8, 3, 2, 0 },
 #if VA_CHECK_VERSION(0, 38, 1)
     { "YUV420_10", VA_RT_FORMAT_YUV420_10BPP, 10, 3, 1, 1 },