Support layer: Force NoContraction on position

[solidpixel]

Variability in gl_Position calculations can break multi-pass rendering where the second pass needs DEPTH_EQUAL, as even minor changes due to floating point reassociation can change the value enough to fail the depth test.

For GLSL shaders can annotate variables with the precise or invariant qualifiers which turns into either NoContract or Invariant decorations in the SPIR-V. We should provide an option to force this for specific shader stages.