Keep each side at 1 or more with fit=outside too

fit=outside works both sides out again from the larger scale, and a side
scaled to 1 could come out just under 1 and round down to 0, which
libvips refuses: a 49x8192 source at 0x100 and a 49x49 source at 1x1
answered 500. README.md now says what fit=outside does, with the 8192x1
source at 100x0 that comes out at 8192x1, and how a 0 side is worked
out from the source's proportions.

Model: opus-5-5

README.md

@@ -536,8 +536,13 @@ Where:
 - `query` — source query string as under Routes, empty string if none, with `%`
   and `:` written as in `path`; neither then holds a `:`, so path `/a:b` with no
   query and path `/a` with query `b:` are signed differently
-- `width` — requested width in pixels, `0` for original
-- `height` — requested height in pixels, `0` for original
+- `width` — requested width in pixels, or `0`: the width is then worked out from
+  the source's proportions and the requested height, rounded down but at least
+  1, or is the source's own width when the height is `0` too (the `orig` size)
+- `height` — requested height in pixels, or `0`: the height is then worked out
+  from the source's proportions and the requested width, rounded down but at
+  least 1, or is the source's own height when the width is `0` too (the `orig`
+  size)
 - `format` — output format, one of those listed under Routes, with `original`
   signed as `orig`, `jpg` as `jpeg`, and no format as `jxl`, so a URL with no
   format has the signature of the same URL ending in `.jxl`; `auto` is signed as
@@ -550,7 +555,10 @@ Where:
   with 400
 - `fit` — the URL's `fit` query parameter (cover, contain, fill, inside,
   outside), or `cover` when the URL has no `fit`; a request whose `fit` is
-  anything else, an empty `fit=` included, is refused with 400
+  anything else, an empty `fit=` included, is refused with 400. `outside` scales
+  the image to cover the requested size, keeping its proportions, so it can come
+  out larger than that size on one side: an 8192x1 source at `100x0`, which is
+  100x1 by its proportions, comes out at 8192x1
 
 The URL's `sig` is the HMAC-SHA256 result in base64url (the URL-safe alphabet of
 RFC 4648) with the trailing `=` padding kept, 44 characters in all. pixa