Fix gles31 frag prim

Merged Dave Airlie requested to merge airlied/virglrenderer:fix-gles31-frag-prim-id into master

GLES 3.1 + EXT_geometry_shader on the host llvmpipe, causes it to crash due to trying to use gl_PrimitiveID in a fragment shader. Fix that, however we ran out of flags, so make req a 64-bit field.

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