Add ClipDistance and CullDistance tests

[EmilioLaiso]

Adds baseline tests for two graphics-stage system values from the
rasterization-coverage epic (#1146).
Both attach a float to a vertex. The rasterizer reads them and decides
whether to draw — but at different granularities:

• SV_ClipDistance: the value is interpolated across the
  primitive. Any pixel whose interpolated value is negative is dropped
  before the pixel shader runs.
• SV_CullDistance: the value is not interpolated. If every
  vertex of a primitive has a negative value, the entire primitive is
  dropped before rasterization.

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ClipDistance.test

A fullscreen quad over a 4×1 render target. The vertex buffer is set up
so that clipDist == NDC.x at every vertex. After linear interpolation
across the quad, every pixel's clipDist equals its own NDC x.

Pixel centers in NDC land at -0.75, -0.25, +0.25, +0.75, so:

• Pixels 0, 1 get a negative clip-distance → clipped, RT stays cleared.
• Pixels 2, 3 get a positive clip-distance → pixel shader runs, writes red.

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CullDistance.test

Two separate triangles drawn in a single call onto a 4×1 RT, each with
a uniform cullDist at all three of its vertices:

• Triangle A covers pixels 0 and 1, cullDist = +1 everywhere → kept,
  pixel shader writes red.
• Triangle B covers pixels 2 and 3, cullDist = -1 everywhere → entire
  primitive dropped before rasterization, pixels stay cleared.