Update CHANGELOG.md

Author: gkjohnson

CHANGELOG.md

@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [0.4.20] - Unreleased
+### Changed
+- ImageOverlayPlugin: Textures are now assumed to be straight alpha.
+
 ## [0.4.19] - 2025.12.19
 ### Changed
 - Moved "GoogleCloudAuthPlugin" to "3d-tiles-renderer/core/plugins".