Test that a side computed for a size with one side 0 is at least 1

An 8192x1 source at 100x0 and 1300x0 and an 8192x2 source at 100x0, and
the same turned on their side, in every fit mode. The test fails until
the fix for #283 lands; TODO.md
records the bug at the top as P1 until then.

Model: opus-5-5

TODO.md

@@ -26,7 +26,10 @@ resolved. The disk cache is now size-bounded with LRU eviction
 
 # Next Step
 
-P2: security: per-origin rate limiting
+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)
 
 # Completed Steps
 
@@ -997,6 +1000,7 @@ P2: security: per-origin rate limiting
 
 # Future Steps
 
+- P2: security: per-origin rate limiting
 - P2: HTTP response handling
     - Last-Modified headers
 - P2: configuration