One image request type instead of five (closes #307)

The new package internal/imagerequest holds the request type, its two
parts Source and Transform, the Format and Fit types, the dimension and
fit checks, the /v1/image/ path parsing and the variant key; it imports
no other package of pixa. imgcache.ParsedURL, imgcache.ImageRequest,
signature.Request, imageprocessor.Request, the second Size, format and
fit types and the conversions between them are gone. The image
processor takes a Transform only. encurl keeps a private struct with the
token's short CBOR field names. VariantKey moves with the function that
makes it. Tests follow the moved and renamed code; the tests of the
removed conversions go with them.

Model: opus-5-5

internal/imageprocessor/source_formats_internal_test.go

@@ -4,6 +4,8 @@ import (
 	"bytes"
 	"io"
 	"testing"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // TestImageProcessor_LoadsEachSourceFormat verifies that an image in each of
@@ -14,11 +16,13 @@ func TestImageProcessor_LoadsEachSourceFormat(t *testing.T) {
 
 	proc := New(Params{})
 
-	for _, format := range []Format{
-		FormatJPEG, FormatPNG, FormatGIF, FormatWebP, FormatAVIF, FormatJXL,
+	for _, format := range []imagerequest.Format{
+		imagerequest.FormatJPEG, imagerequest.FormatPNG, imagerequest.FormatGIF,
+		imagerequest.FormatWebP, imagerequest.FormatAVIF, imagerequest.FormatJXL,
 	} {
 		saved, err := proc.Process(t.Context(),
-			bytes.NewReader(createTestPNG(t, 20, 20)), &Request{Format: format})
+			bytes.NewReader(createTestPNG(t, 20, 20)),
+			&imagerequest.Transform{Format: format})
 		if err != nil {
 			t.Fatalf("saving the test image as %s: %v", format, err)
 		}
@@ -29,7 +33,7 @@ func TestImageProcessor_LoadsEachSourceFormat(t *testing.T) {
 		}
 
 		result, err := proc.Process(t.Context(), bytes.NewReader(source),
-			&Request{Format: FormatOriginal})
+			&imagerequest.Transform{Format: imagerequest.FormatOriginal})
 		if err != nil {
 			t.Errorf("orig of the %s image: %v", format, err)