Turned on SkSL->GLSL compiler

src/sksl/sksl_frag.include

 STRINGIFY(
 
 // defines built-in interfaces supported by SkiaSL fragment shaders
 
 layout(builtin=15) in vec4 gl_FragCoord;
 
+// 9999 is a temporary value that causes us to ignore these declarations beyond
+// adding them to the symbol table. This works fine in GLSL (where they do not
+// require any further handling) but will fail in SPIR-V. We'll have a better
+// solution for this soon.
+layout(builtin=9999) vec4 gl_LastFragData[1];
+layout(builtin=9999) vec4 gl_LastFragColor;
+layout(builtin=9999) vec4 gl_LastFragColorARM;
+layout(builtin=9999) int gl_SampleMaskIn[];
+layout(builtin=9999) out int gl_SampleMask[];
+layout(builtin=9999) vec4 gl_SecondaryFragColorEXT;
+
+layout(location=0,index=0,builtin=10001) out vec4 sk_FragColor;
+
 )
-