Refuse, before decoding, input libvips would not load as a source format

govips picks the libvips loader for the input with DetermineImageType.
The image processor now names the format from that type before decoding,
and refuses with ErrUnsupportedInputFormat, answered 415, any input it
cannot name as one of the six source formats. An SVG whose first bytes
pass the first-bytes check for AVIF therefore never reaches librsvg.
orig is the format named there. README.md and TODO.md say so.

Model: opus-5-5

README.md

@@ -191,10 +191,10 @@ path under `/v1/` answers 200, in maintenance mode too.
   redirects to, is `localhost`, ends in `.localhost` or `.local`, or has an
   address in a blocked network (see `blocked_networks`); 429 over
   `image_requests_per_minute` (below); 415 when the upstream answers with a
-  `Content-Type` that is not one of the source formats (below), or for `orig` of
-  a source pixa cannot save in its own format; 502 when the upstream answered
-  with an error status, and for 5 minutes after that for the same source URL;
-  503 when pixa is busy or in maintenance mode; 500 for any other failure.
+  `Content-Type` that is not one of the source formats (below), or with an image
+  libvips would not load as one of them; 502 when the upstream answered with an
+  error status, and for 5 minutes after that for the same source URL; 503 when
+  pixa is busy or in maintenance mode; 500 for any other failure.
 - `GET` or `HEAD` `/v1/e/<token>/<name>` — an image through an encrypted URL
   (see Encrypted URLs). Needs: nothing but the URL. Answers: 200; 304 when
   `If-None-Match` matches the image's `ETag`; 400 for a token that does not
@@ -265,7 +265,8 @@ once, is refused with 400.
 The source image may be JPEG, PNG, GIF, WebP, AVIF or JPEG XL. The upstream's
 `Content-Type` must name its format, and the image's first bytes must match it.
 An upstream answering with any other `Content-Type`, SVG included, is refused
-with 415.
+with 415, and so is an image that libvips would not load as one of these
+formats, such as an SVG whose first bytes pass the check for AVIF.
 
 With the format `auto`, pixa chooses the format for each request from its
 `Accept` header, in this order: