Turned on SkSL->GLSL compiler

src/sksl/README

+Overview
+========
+
+SkSL ("Skia Shading Language") is a variant of GLSL which is used as Skia's 
+internal shading language. SkSL is, at its heart, a single standardized version
+of GLSL which avoids all of the various version and dialect differences found
+in GLSL "in the wild", but it does bring a few of its own changes to the table.
+
+Skia uses the SkSL compiler to convert SkSL code to GLSL, GLSL ES, or SPIR-V
+before handing it over to the graphics driver.
+
+Differences from GLSL
+=====================
+
+SkSL is based on GLSL 4.5. For the most part, write SkSL exactly as you would
+desktop GLSL, and the SkSL compiler will take care of version and dialect
+differences (for instance, you always use "in" and "out", and skslc will handle
+translating them to "varying" and "attribute" as appropriate). Be aware of the 
+following differences between SkSL and GLSL:
+
+* no #version or "precision" statement is required, and they will be ignored if
+  present
+* the output color is sk_FragColor (do not declare it)
+* lowp, mediump, and highp are always permitted (but will only be respected if 
+  you run on a GLES device)
+* you do not need to include ".0" to make a number a float (meaning that
+  "vec2(x, y) * 4" is perfectly legal in SkSL, unlike GLSL where it would often
+  have to be expressed "vec2(x, y) * 4.0". There is no performance penalty for 
+  this, as the number is converted to a float at compile time)
+* type suffixes on numbers (1.0f, 0xFFu) are both unnecessary and unsupported
+* some built-in functions and one or two rarely-used language features are not
+  yet supported (sorry!)
+
+SkSL is still under development, and is expected to diverge further from GLSL
+over time.