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 reads a cached source's format from its file's first bytes, as it
reads a variant's. The proxy hands the format to Process. Process
refuses a format that is not a source format, and 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/imageprocessor/orig_format_internal_test.go

@@ -10,8 +10,9 @@ import (
 )
 
 // TestImageProcessor_OrigOfUnnamedFormatRefused verifies that orig of a TIFF
-// input, which is not a source format, is refused with
-// ErrUnsupportedInputFormat before it is decoded, and not saved as JPEG.
+// input is refused with ErrUnsupportedInputFormat before it is decoded, and
+// not saved as JPEG. TIFF is not a source format, and the test passes the
+// input to Process as a JPEG.
 func TestImageProcessor_OrigOfUnnamedFormatRefused(t *testing.T) {
 	t.Parallel()
 
@@ -31,7 +32,7 @@ func TestImageProcessor_OrigOfUnnamedFormatRefused(t *testing.T) {
 		MaxInputBytes:           50 << 20,
 		MaxConcurrentProcessing: 1,
 	}).Process(
-		t.Context(), bytes.NewReader(tiff),
+		t.Context(), bytes.NewReader(tiff), imagerequest.FormatJPEG,
 		&imagerequest.Transform{Format: imagerequest.FormatOriginal},
 	)
 	if result != nil {