Support SVG sources (closes #68)

The fetcher accepts image/svg+xml and internal/magic detects SVG. orig
of an SVG at the orig size is the SVG unchanged, cached as a variant; at
another size it is ErrSVGNotOriginalSize (400). Any other format renders
it with libvips' thumbnail at the least size covering the output, at
most 8192 on a side, so it stays sharp. An SVG's own size is not
limited; one with a side over 8192 times the other is ErrSVGTooThin
(415). The cache reads up to 1024 first bytes to find a file's format.
Tests that pinned the SVG refusal now expect SVG to be read. Render time
and memory are unbounded: P1 in TODO.md until the owner decides on
#68.

Model: opus-5-5

internal/imageprocessor/source_formats_internal_test.go

@@ -51,7 +51,7 @@ func TestImageProcessor_LoadsEachSourceFormat(t *testing.T) {
 }
 
 // TestImageProcessor_NotASourceFormatRefused verifies that a source format
-// other than the six, such as orig, auto or none, is refused with
+// other than the seven, such as orig, auto or none, is refused with
 // ErrUnsupportedInputFormat. The input is text, which libvips reads as no
 // format.
 func TestImageProcessor_NotASourceFormatRefused(t *testing.T) {