Support SVG sources (closes #68)

The fetcher accepts image/svg+xml; internal/magic detects SVG. orig of
an SVG at the orig size is the SVG unchanged, cached, with a
Content-Security-Policy allowing only its styles and data: images; at
another size, ErrSVGNotOriginalSize (400). Any other format renders it
at the least size covering the output, at most 8192 on a side.
Rendering refuses an SVG with a side over 8192 times the other
(ErrSVGTooThin, 415), and the orig size of one above 8192 on a side
(ErrSVGTooLargeForOrigSize, 400). Child processes of pixad read an
SVG's size and render it, killed after 20 seconds and limited to 1 GiB;
any failure there is ErrSVGRenderFailed (415). Tests pinning the SVG
refusal now expect SVG.

Model: opus-5-5

internal/imagerequest/imagerequest.go

@@ -14,7 +14,8 @@ import (
 var ErrInvalidFitMode = errors.New("invalid fit mode")
 
 // Format is an image format. pixa reads and writes the formats from JPEG to
-// GIF below. In a request, orig and auto stand for one of them.
+// GIF below. It reads SVG and writes it only as orig of an SVG source. In a
+// request, orig and auto stand for another format.
 type Format string
 
 // Supported image formats.
@@ -26,6 +27,7 @@ const (
 	FormatAVIF     Format = "avif"
 	FormatJXL      Format = "jxl"
 	FormatGIF      Format = "gif"
+	FormatSVG      Format = "svg"
 
 	// FormatAuto stands for JPEG XL, AVIF, WebP or JPEG, chosen for each
 	// request from its Accept header once the URL's signature or token has
@@ -50,6 +52,8 @@ func (f Format) MIMEType() string {
 		return "image/jxl"
 	case FormatGIF:
 		return "image/gif"
+	case FormatSVG:
+		return "image/svg+xml"
 	case FormatOriginal, FormatAuto:
 		return ""
 	default: