Detect a fetched image's format from its bytes once (closes #319)

magic.DetectFormat returns an imagerequest.Format. ValidateMagicBytes
returns the format it checked against the upstream Content-Type. The
cache stores that format in the source's row, so a variant made from a
cached source uses it too. The proxy hands it to Process, which still
refuses input libvips would load with another loader, such as an SVG
passing as AVIF. Format.MIMEType declares each MIME string once, and the
fetcher, the cache's variant read and the Accept matching use it.
magic.MIMEType, magic.ImageFormat, MIMEToImageFormat, magic's SVG
detection, detectFormat, formatFromString, FormatToMIME and
Result.InputFormat are gone.

Model: opus-5-5

internal/imageproxy/negative_cache_internal_test.go

@@ -55,7 +55,7 @@ func TestProxy_Get_ServesCachedVariantOfNegativeCachedURL(t *testing.T) {
 
 	source := photoVariant(fixtures, 85, imagerequest.FitCover).Source
 
-	reader, _, err := svc.cache.Source(t.Context(), source)
+	reader, _, _, err := svc.cache.Source(t.Context(), source)
 	if err != nil {
 		t.Fatalf("Source() error = %v, want the cached source", err)
 	}