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

The side was rounded down, so for a thin source it came out at 0, and
libvips made the size square: an 8192x1 source at 100x0 came out at
100x100, at 1300x0 with fit=cover libvips asked for about 39 GB, and an
8192x2 source at 100x0 answered 500. fit=contain and fit=inside work
both sides out again from the smaller scale and keep each at 1 or more
as well, as a 1x8192 source at 0x100 answered 500 there.

Model: opus-5-5

TODO.md

@@ -26,13 +26,21 @@ resolved. The disk cache is now size-bounded with LRU eviction
 
 # Next Step
 
-P1: security: a size with one side `0` whose side computed from the source's
-proportions rounds down to 0 comes out square, can make libvips ask for about 39
-GB (an 8192x1 source at `1300x0`), or answers 500
-(https://git.eeqj.de/sneak/pixa/issues/283)
+P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-16 a size with one side `0` keeps the side computed from the source's
+  proportions at 1 or more (closes #283): it was rounded down, so for a thin
+  source it came out at 0, and libvips then made the size square. An 8192x1
+  source at `100x0` came out at 100x100, and at `1300x0` with `fit=cover` made
+  libvips ask for about 39 GB at once; an 8192x2 source at `100x0` answered 500.
+  `fit=contain` and `fit=inside` work both sides out again from the smaller of
+  the two scales, and keep each at 1 or more too, so a 1x8192 source at `0x100`
+  no longer answers 500 there. An 8192x1 source at `1300x0` now comes out at
+  1300x1, and the process running it alone peaked at about 34 MiB resident. With
+  `fit=outside` the image is scaled to cover the size, so that source at `100x0`
+  comes out at 8192x1.
 - 2026-10-15 a thin source requested with `fit=cover` no longer makes libvips
   build a huge image (closes #282): libvips' thumbnail scaled the whole source
   to cover the requested size and copied that into memory before it cut out the
@@ -1000,7 +1008,6 @@ GB (an 8192x1 source at `1300x0`), or answers 500
 
 # Future Steps
 
-- P2: security: per-origin rate limiting
 - P2: HTTP response handling
     - Last-Modified headers
 - P2: configuration