avif: performance and quality improvements

Enable decoder to use more threads.
8bit YUV->RGB conversion is significantly faster
when AVIF_RGB_FORMAT_RGBA format is used,
because libavif can use libyuv to perform conversion quickly.
Prefer faster AVIF_CHROMA_UPSAMPLING_FASTEST when decoding animation.
Encoder speed changed from to 8 to 7.
Recent AV1 encoders got faster,
so it is OK to switch to better compression quality.
Remove obsolete image/avif-sequence mime-type
as it was merged with image/avif.

src/imageformats/avif.json

@@ -1,4 +1,4 @@
 {
     "Keys": [ "avif", "avifs" ],
-    "MimeTypes": [ "image/avif", "image/avif-sequence" ]
+    "MimeTypes": [ "image/avif", "image/avif" ]
 }