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 CBOR field names, so tokens made before still decode. VariantKey
moves with the function that makes it. Tests follow the moved code; the
tests of the removed conversions go with them.

Model: opus-5-5

internal/imageprocessor/max_concurrent_processing_internal_test.go

@@ -11,6 +11,8 @@ import (
 	"testing"
 	"testing/iotest"
 	"time"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // errTestReadFailed is the error the unreadable test input returns.
@@ -77,12 +79,12 @@ func (r *gatedReader) Read(p []byte) (int, error) {
 }
 
 // smallJPEGRequest asks for a 5x5 JPEG.
-func smallJPEGRequest() *Request {
-	return &Request{
-		Size:    Size{Width: 5, Height: 5},
-		Format:  FormatJPEG,
+func smallJPEGRequest() *imagerequest.Transform {
+	return &imagerequest.Transform{
+		Size:    imagerequest.Size{Width: 5, Height: 5},
+		Format:  imagerequest.FormatJPEG,
 		Quality: 85,
-		FitMode: FitCover,
+		FitMode: imagerequest.FitCover,
 	}
 }
 
@@ -269,7 +271,7 @@ func TestProcessReleasesSlotOnError(t *testing.T) {
 	cases := []struct {
 		name  string
 		input io.Reader
-		req   *Request
+		req   *imagerequest.Transform
 		// want is the error Process must return; nil means any error.
 		want error
 	}{