initial checkin of SkSL compiler

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1984363002
CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Ubuntu-GCC-x86_64-Release-CMake-Trybot,Build-Mac-Clang-x86_64-Release-CMake-Trybot

Review-Url: https://codereview.chromium.org/1984363002
diff --git a/src/sksl/sksl_frag.include b/src/sksl/sksl_frag.include
new file mode 100644
index 0000000..b4047fe
--- /dev/null
+++ b/src/sksl/sksl_frag.include
@@ -0,0 +1,7 @@
+STRINGIFY(
+
+// defines built-in interfaces supported by SkiaSL fragment shaders
+
+layout(builtin=15) in vec4 gl_FragCoord;
+
+)
\ No newline at end of file