Claim only one libvips worker thread per image in initVips

The comment said the images processed at once use no more CPUs than
max_concurrent_processing, which the review found does not hold for GIF
output. It now says only that libvips runs one worker thread per image.

Model: opus-5-5

internal/imageprocessor/imageprocessor.go

@@ -20,11 +20,10 @@ import (
 var vipsOnce sync.Once
 
 // initVips initializes libvips with quiet logging, one worker thread per
-// image and no operation cache. With one thread each, the images processed
-// at once use no more CPUs than MaxConcurrentProcessing. Each request
-// decodes different source bytes, so the operation cache would rarely be
-// hit and would hold memory outside MaxConcurrentProcessing; repeated
-// requests are served from pixa's disk cache instead.
+// image and no operation cache. Each request decodes different source
+// bytes, so the operation cache would rarely be hit and would hold memory
+// outside MaxConcurrentProcessing; repeated requests are served from
+// pixa's disk cache instead.
 func initVips() {
 	vipsOnce.Do(func() {
 		vips.LoggingSettings(nil, vips.LogLevelError)