Give the slowest AVIF encode at the limit across quality

The AVIF times given before, 25 seconds with an alpha channel and 15
without, came from quality 85 and 100 only. Measured again with the
runtime image's libvips on one thread at qualities 1 to 100, 4096x4096,
8192x2048 and 2048x8192 random pixels take up to about 34 seconds with
an alpha channel and 22 without, the slowest at quality 90 to 99. That
is under 40, so the limit stays at 16,777,216 pixels. The doc comments,
README.md and TODO.md now give those times. WebP and JPEG XL, measured
the same way, stay within the times already given, which now say they
hold at any quality. The TODO.md entry moves to the top of Completed
Steps.

Model: opus-5-5

internal/imageprocessor/imageprocessor.go

@@ -145,9 +145,10 @@ var ErrOutputTooLarge = errors.New("output image dimensions exceed maximum")
 // MaxAVIFOutputPixels is the most pixels an AVIF output may have, so that an
 // encode takes at most 40 seconds of the default downstream_timeout of 60
 // seconds; AVIF is already at the lowest effort govips can set (avifEffort).
-// With one thread, as pixad runs libvips, saving 4096x4096 or 8192x2048
-// random pixels with an alpha channel takes up to about 25 seconds, and
-// 5120x4096 took 39.
+// With one thread, as pixad runs libvips, saving 4096x4096, 8192x2048 or
+// 2048x8192 random pixels with an alpha channel takes up to about 34
+// seconds, the slowest at any quality and at 8 or 16 bits per sample, and
+// 5120x4096 at 8 bits already takes about 36.
 const MaxAVIFOutputPixels = 4096 * 4096
 
 // ErrAVIFOutputTooLarge is returned, before anything is decoded, when an AVIF
@@ -680,8 +681,8 @@ func exportGIF(img *vips.ImageRef) ([]byte, error) {
 // default of 4 so that an encode takes at most 40 seconds of the default
 // downstream_timeout of 60 seconds. With one thread, as pixad runs libvips,
 // saving an 8192x8192 image of random pixels with an alpha channel takes up
-// to about 23 seconds at 2, and up to about 16 without one; with an alpha
-// channel, 3 takes about 50 seconds and 4 about 61.
+// to about 23 seconds at 2, and up to about 16 without one, at any quality;
+// with an alpha channel, 3 takes about 50 seconds and 4 about 61.
 const webpEffort = 2
 
 // exportWebP encodes img as lossy WebP at quality and webpEffort, without
@@ -697,12 +698,13 @@ func exportWebP(img *vips.ImageRef, quality int) ([]byte, error) {
 }
 
 // avifEffort is the AVIF effort, from 0 (fastest) to 9; 1 is the lowest
-// govips can set. With one thread, as pixad runs libvips, 1 takes about 58
-// seconds to save an 8192x8192 RGB image of random pixels, and about 99
-// with an alpha channel, against the default downstream_timeout of 60
-// seconds, so an AVIF output is held to MaxAVIFOutputPixels. On an image of
-// milder noise, which 1 saves in about 12 seconds, 2 takes nearly a minute
-// and libvips' default, 4, takes minutes.
+// govips can set. With one thread, as pixad runs libvips, 1 takes about 52
+// seconds to save an 8192x8192 RGB image of random pixels at quality 85,
+// and about 96 with an alpha channel, against the default
+// downstream_timeout of 60 seconds, so an AVIF output is held to
+// MaxAVIFOutputPixels. On an image of milder noise, which 1 saves in about
+// 12 seconds, 2 takes nearly a minute and libvips' default, 4, takes
+// minutes.
 const avifEffort = 1
 
 // avifBitdepth is the AVIF bit depth, 8 bits per sample for every image.
@@ -733,8 +735,8 @@ const jxlResolution = 72 / 25.4
 // libvips' default of 7 so that an encode takes at most 40 seconds of the
 // default downstream_timeout of 60 seconds. With one thread, as pixad runs
 // libvips, saving an 8192x8192 image of random pixels with an alpha channel
-// takes up to about 27 seconds at 4, and up to about 13 without one; with an
-// alpha channel, 5 takes about 43 seconds and 7 about 81.
+// takes up to about 27 seconds at 4, and up to about 13 without one, at any
+// quality; with an alpha channel, 5 takes about 43 seconds and 7 about 81.
 const jxlEffort = 4
 
 // exportJXL encodes img as JPEG XL at quality and jxlEffort, without