[skotty,sksg] Initial matte layer support

TBR=
Change-Id: I5b689f5d7b0d147fa200cf5bffe476077085cb19
Reviewed-on: https://skia-review.googlesource.com/93300
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 7f7a6dc..255bef9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1384,6 +1384,7 @@
       "experimental/sksg/SkSGNode.cpp",
       "experimental/sksg/SkSGPaintNode.cpp",
       "experimental/sksg/SkSGRenderNode.cpp",
+      "experimental/sksg/effects/SkSGMaskEffect.cpp",
       "experimental/sksg/effects/SkSGOpacityEffect.cpp",
       "experimental/sksg/effects/SkSGTransform.cpp",
       "experimental/sksg/geometry/SkSGMerge.cpp",