Update for v4.2.1.

Author: MarkCallow

Diff for: RELEASE_NOTES.md

@@ -2,6 +2,23 @@
 <!-- SPDX-License-Identifier: Apache-2.0 -->
 Release Notes
 =============
+## Version 4.2.1
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+### Changes since v4.2.0 (by part)
+### libktx
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+* Fix memory leak of input\_image in ktxTexture2\_CompressAstcEx (fa1fe4d7) (@null)
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 ## Version 4.2.0
 ### Overview