sketch an skvm

With all the thinking around a stack-based interpreter,
I figured I'd sketch out some ideas for a register VM too.

I kind of have the hunch that this is the direction that
will actually let us replace large amounts of Skia's CPU
backend with an efficient interpreter or JIT.

Change-Id: Ia2b5ba4a3fc27556f5b6ba95cd1ace46d3217403
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/216665
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 160a016..e9046ea 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -250,6 +250,7 @@
   "$_tests/SkShaperJSONWriterTest.cpp",
   "$_tests/SkSharedMutexTest.cpp",
   "$_tests/SkUTFTest.cpp",
+  "$_tests/SkVMTest.cpp",
   "$_tests/SkVxTest.cpp",
   "$_tests/Skbug5221.cpp",
   "$_tests/Skbug6389.cpp",