Add kRG_1616 and kAlpha_16 SkColorTypes

This also switches GrColorType::kR_16 to kAlpha_16 to more closely match raster.

Bug: skia:9121
Change-Id: I03c6e6c52c90aa4223478c5ea6c8b2ed8558f677
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/239930
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/RELEASE_NOTES.txt b/RELEASE_NOTES.txt
index bdf986d..6a72f3f 100644
--- a/RELEASE_NOTES.txt
+++ b/RELEASE_NOTES.txt
@@ -92,8 +92,12 @@
     https://review.skia.org/238856
 
   * Added kRG_88_SkColorType. This is intended to help support YUV uses case (e.g., NV12).
-    As such, it the addition is focused on allowing creation of SkPixmaps and SkImages and not
+    As such, the addition is focused on allowing creation of SkPixmaps and SkImages and not
     SkSurfaces (i.e., who wants to render to RG?)
 
   * Add GrContext::precompileShader to allow up-front compilation of previously-cached shaders.
     https://review.skia.org/239438
+
+  * Added kAlpha_16_SkColorType and kRG_1616_SkColorType. This is intended to help support HDR YUV
+    uses case (e.g., P010 and P016). As such, the addition is focused on allowing creation of
+    SkPixmaps and SkImages and not SkSurfaces (i.e., who wants to render to render to these?)