Invariant related processing.

* Fix a bug in PreProcessor for STDGL pragma.
* Record all invariant settings and set them in ShaderVariable.
* Write #pragma STDGL invariant(all) in GL

BUG=angle:776
TEST=https://www.khronos.org/registry/webgl/sdk/tests/conformance/glsl/misc/shaders-with-invariance.html

Change-Id: Ie28b75480deed79f0c9f26e3b98f1778d1290182
Reviewed-on: https://chromium-review.googlesource.com/223610
Tested-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>