Upgrade to libvips 8.10.6-alpha1

- Prebuilt binaries now include mozjpeg and libimagequant (BSD 2-Clause)
- Prebuilt binaries limit AVIF support to the most common 8-bit depth
- Add `mozjpeg` option to `jpeg` method, sets mozjpeg defaults
- Reduce the default PNG `compressionLevel` to the more commonly used 6

src/pipeline.h

@@ -258,7 +258,7 @@ struct PipelineBaton {
     jpegOptimiseScans(false),
     jpegOptimiseCoding(true),
     pngProgressive(false),
-    pngCompressionLevel(9),
+    pngCompressionLevel(6),
     pngAdaptiveFiltering(false),
     pngPalette(false),
     pngQuality(100),