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/imageproxy/max_concurrent_processing_internal_test.go

@@ -41,7 +41,8 @@ func holdProcessingSlot(
 	input, feed := io.Pipe()
 
 	go func() {
-		_, _ = proc.Process(t.Context(), input, &imagerequest.Transform{})
+		_, _ = proc.Process(t.Context(), input, imagerequest.FormatJPEG,
+			&imagerequest.Transform{})
 	}()
 
 	_, err := feed.Write([]byte{0})
@@ -57,10 +58,11 @@ func holdProcessingSlot(
 
 // TestProxy_Get_WaitsForSlotBeforeReadingCachedSource checks that a
 // request whose source is cached holds none of it while it waits for a
-// processing slot: it reads the cached file only once it has a slot. With
-// the only slot held, a request for a new width of the cached 100x100 photo
-// waits; the cached file is then rewritten as a 100x50 image before the slot
-// is freed, so the request must answer with that image scaled to 40x20.
+// processing slot: past the first bytes, which give the source's format, it
+// reads the cached file only once it has a slot. With the only slot held, a
+// request for a new width of the cached 100x100 photo waits; the cached file
+// is then rewritten as a 100x50 image before the slot is freed, so the
+// request must answer with that image scaled to 40x20.
 func TestProxy_Get_WaitsForSlotBeforeReadingCachedSource(t *testing.T) {
 	t.Parallel()
 
@@ -79,7 +81,7 @@ func TestProxy_Get_WaitsForSlotBeforeReadingCachedSource(t *testing.T) {
 
 	cachedSource := widthOnlyRequest(fixtures, 50).Source
 
-	reader, _, err := svc.cache.Source(t.Context(), cachedSource)
+	reader, _, _, err := svc.cache.Source(t.Context(), cachedSource)
 	if err != nil {
 		t.Fatalf("Source() error = %v; want the cached source", err)
 	}