Fix output GLSL for primitive ID in a geometry shader (#1214)

We had been translating an `SV_PrimitiveID` input in a shader over to `gl_PrimitiveID` in GLSL.
That translation seemed to work just fine for users, so we thought it was correct.

It turns out that `gl_PrimitiveID` is the correct GLSL for a primitive ID input in every stage *except* a geometry shader.
In a geometry shader, `gl_PrimitiveID` is a primitive ID *output*, and if you want the input case you have to write `gl_PrimitiveIDIn` (note the `In` suffix).

This change sets aside my bewilderment at the above long enough to implement a workaround in the GLSL legalization step.
I also modified our current geometry shader cross compilation test to make use of an input primitive ID.

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