Serve the format auto, chosen from the Accept header (closes #88)

auto is a format in the /v1/image/ path, an encrypted URL's token and
the generator page. Once the signature or token is checked, pixa
chooses AVIF when Accept names image/avif, else WebP when it names
image/webp, else JPEG when the most specific of image/jpeg, image/* and
*/* allows it or Accept is absent. q=0 refuses a format; a header
allowing none of the three answers 406, one that does not parse 400.
The signature and token cover auto itself; the cache key and ETag use
the chosen format. Answers from then on carry Vary: Accept.

Model: opus-5-5

internal/imgcache/imgcache.go

@@ -22,6 +22,11 @@ const (
 	FormatWebP     ImageFormat = "webp"
 	FormatAVIF     ImageFormat = "avif"
 	FormatGIF      ImageFormat = "gif"
+
+	// FormatAuto stands for AVIF, WebP or JPEG, chosen for each request
+	// from its Accept header once the URL's signature or token has been
+	// checked; it is never processed or cached as itself.
+	FormatAuto ImageFormat = "auto"
 )
 
 // Size represents requested image dimensions