Compare shader validation options against gecko's choices

[jdm]

https://searchfox.org/mozilla-central/rev/d69ec052bed8700af7a283e37b60b4af22734930/dom/canvas/WebGLShaderValidator.cpp#31

There are some differences that we probably want to copy.

[asajeffrey]

This is causing some macOS failures, e.g. /_webgl/conformance/glsl/misc/uninitialized-local-global-variables.html caused by not setting the `SH_INITIALIZE_UNINITIALIZED_LOCALS flag.

These failures don't show up in CI because there we're testing with --headless, which uses OSMesa.

Chat on #servo at https://chat.mozilla.org/#/room/#servo:mozilla.org/$Q9Tpvp1DwAuva_GPOGLlnDb_aZLauvvzXzvzMC46nQY