[Impeller] Fix perspective clips with a large perspective bias

[flar]

Fixes: #173104

We were precomputing the perspective adjustment inside the clipping vertex shader, which prevented the GPU from performing correct clipping against the camera plane.

[gemini-code-assist]

Code Review

This pull request fixes incorrect clipping for geometries under a perspective transform with a large bias. The main change is in the clip.vert shader, which now correctly lets the GPU handle perspective division and clipping by not dividing gl_Position by w in the vertex shader. Instead, it sets gl_Position.z relative to w. Complementing this, FillRectGeometry::GetCoverage is updated to use TransformAndClipBounds for accurate coverage calculation with perspective transforms. A new playground test is also added for visual verification. The changes are correct and address the issue effectively.

[gaaclarke]

LGTM. The math should be equivalent. Previously the weight was set to 1 by applying the division, then undoing that for the z component to maintain the clip depth. This works similarly but keeps the weight value intact by just multiplying it by the weight instead. My intuition here is that we are getting different results by delaying the weight division instead of doing it here.