Support SVG sources (closes #68)

The fetcher accepts image/svg+xml again and internal/magic detects SVG.
A raster format renders the SVG at the least size that covers the
output, held to 8192 on a side, through libvips' thumbnail, which
renders at any scale, so an enlarged SVG stays sharp. An SVG's own size
is not limited; one with a side more than 8192 times the other is
ErrSVGTooThin, answered 415. orig of an SVG at the orig size is the SVG
unchanged, cached as a variant; at any other size it is
ErrSVGNotOriginalSize, answered 400. The cache reads up to 1024 first
bytes of a file to find its format. Render time and memory grow with
what the SVG draws; their bound is the owner's decision on #68. Tests that pinned SVG as refused now
expect it to be read.

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: