Add --layered flag to avifenc

Add configurable progressive encoding support to avifenc through
layered images with --layered flag.
Add --scaling-mode:u flag to set scaling mode of each layer.

Remove alpha quality change in --progressive.

CHANGELOG.md

@@ -28,6 +28,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Add avifenc flag suffixes ":update" and ":u". Quality-relative,
   tiling-relative and codec-specific flags can now be positional, relative to
   input files.
+* Add experimental support for layered AVIF encoding in avifenc.
+  Use the --layered flag to enable layered AVIF encoding.
+  Layered AVIF has multiple layers, which works like frame of animated AVIF, 
+  and layers can be rendered in progressive manner on supported viewers
+  (e.g. Chrome 94 or newer).
+  Only aom supports layered AVIF encoding at the time of writing.
+  Add --scaling-mode flag to set scaling mode of each layer.
+  This part of AV1 encoder is not as thoroughly tested, so there are higher
+  possibility encoder may crash when given certain configuration or input.
 
 ### Changed
 * Update aom.cmd: v3.7.0