Keep WebP, AVIF and JPEG XL encodes at the limits under 40 seconds (closes #272)

WebP is saved at effort 2 and JPEG XL at effort 4, and an AVIF output of
more than 16,777,216 pixels is refused with 400 before anything is
decoded, as AVIF is already at the lowest effort govips can set. With one
libvips thread, 8192x8192 random pixels with an alpha channel take up to
about 23 seconds as WebP and 27 as JPEG XL, and 4096x4096 as AVIF about
25. Resizing with fit=cover already decodes the input, so the output's
dimensions are now worked out before resizing, in outputDimensions.
Reading the input moves into readInput to keep Process within the
linter's limits.

Model: opus-5-5

internal/handlers/image.go

@@ -264,6 +264,14 @@ func (s *Handlers) respondImageError(
 		return
 	}
 
+	if errors.Is(err, imageprocessor.ErrAVIFOutputTooLarge) {
+		s.respondError(w, fmt.Sprintf("size too large for AVIF: the output "+
+			"would be above %d pixels", imageprocessor.MaxAVIFOutputPixels),
+			http.StatusBadRequest)
+
+		return
+	}
+
 	s.respondError(w, "internal error", http.StatusInternalServerError)
 }
 

internal/handlers/imageenc.go

@@ -170,6 +170,10 @@ func (s *Handlers) handleImageError(w http.ResponseWriter, err error) {
 		s.respondError(w, fmt.Sprintf("size too large for the source: the side "+
 			"computed from its proportions is above %d",
 			imageprocessor.MaxOutputDimension), http.StatusBadRequest)
+	case errors.Is(err, imageprocessor.ErrAVIFOutputTooLarge):
+		s.respondError(w, fmt.Sprintf("size too large for AVIF: the output "+
+			"would be above %d pixels", imageprocessor.MaxAVIFOutputPixels),
+			http.StatusBadRequest)
 	default:
 		s.log.Error("image request failed", "error", err)
 		s.respondError(w, "internal error", http.StatusInternalServerError)