images: Make AVIF chroma subsampling content-aware via the hint

Encode photo/picture hints (and the default) as YUV420 instead of YUV444,
keeping 444 for text/icon/drawing. Lossless stays 444.

This roughly halves the encoder's peak memory (42 -> 27 MiB/MP) and the
output size, while 444 remains available for sharp-edged content. A
3000x3000 image now needs ~239 MiB to encode, down from ~381 MiB (which
sat right at the 384 MiB WASM cap).

Closes #14987

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

resources/images/codec.go

@@ -144,6 +144,7 @@ func (d *Codec) EncodeTo(conf ImageConfig, w io.Writer, img image.Image) error {
 			"compression":  conf.Compression,
 			"quality":      conf.Quality,
 			"encoderSpeed": conf.EncoderSpeed,
+			"hint":         conf.Hint,
 		}
 		return d.avif.Encode(w, img, opts)
 	case WEBP:

resources/images/config.go

@@ -104,7 +104,7 @@ var compressionMethods = map[string]bool{
 	"lossless": true,
 }
 
-// These encoding hints are currently only relevant for Webp.
+// These encoding hints are used by Webp (preset) and Avif (chroma subsampling).
 var hints = map[string]bool{
 	"picture": true,
 	"photo":   true,
@@ -429,7 +429,7 @@ type ImageConfig struct {
 	BgColor color.Color
 
 	// Hint about what type of picture this is. Used to optimize encoding
-	// when target is set to webp.
+	// when target is webp (preset) or avif (chroma subsampling).
 	Hint string
 
 	Compression string
@@ -510,7 +510,7 @@ type ImagingConfig struct {
 	ResampleFilter string
 
 	// Hint about what type of image this is.
-	// Currently only used when encoding to Webp.
+	// Used when encoding to Webp (preset) and Avif (chroma subsampling).
 	// Default is "photo".
 	// Valid values are "picture", "photo", "drawing", "icon", or "text".
 	// Moved to WebpConfig in v0.155.0, but kept here for backwards compatibility.