Enable HLSL legalization

Also added known-good mechanism to fetch latest validated spirv-tools.
Also added -Od and -Os to disable optimizer and optimize for size.

Fetching spirv-tools is optional for both glsl and hlsl. Legalization
of hlsl is done by default if spirv-opt is present at cmake time.
Optimization for glsl is currently done through the option -Os.

Legalization testing is currently only done on four existing shaders.
A separate baseLegalResults directory holds those results. All previous
testing is done with the optimizer disabled.

.travis.yml

@@ -55,6 +55,7 @@ install:
 
 before_script:
   - git clone --depth=1 https://github.com/google/googletest.git External/googletest
+  - update_glslang_sources.py
 
 script:
   - mkdir build && cd build