Keep a side computed for a size with one side 0 at 1 or more (closes #283)

A size with one side 0 has that side worked out from the source's
proportions by integer division, and nothing kept it at 1 or more. When
it rounded down to 0, libvips was asked for a square: an 8192x1 source
at 100x0 came out 100x100, at 1300x0 it asked for one block of about
39 GB, and some shapes answered 500. Each computed side, and each side
contain, inside and outside work out again from the scale, is now at
least 1. README.md now says what fit=outside does and what a 0 side
means; with fit=outside an 8192x1 source at 100x0 comes out 8192x1, as
covering the size requires.

Model: opus-5-5

internal/imgcache/urlparser.go

@@ -29,7 +29,9 @@ var (
 const MaxDimension = imageprocessor.MaxOutputDimension
 
 // ValidateDimension checks one requested width or height; name ("width" or
-// "height") appears in the error. 0 means "original size" and is valid.
+// "height") appears in the error. 0 is valid: that side is then worked out from
+// the source's proportions and the other requested side, rounded down but at
+// least 1, or is the source's own side when the other side is 0 too.
 func ValidateDimension(name string, value int) error {
 	if value < 0 {
 		return fmt.Errorf("%w: %s is negative", ErrInvalidSize, name)