overdraw render feature

Draws the overdraw capture view: how many fragments each pixel cost.

A geometry pass over @geometry.all blends a whole unit additively into a private float target with the depth test switched off, so each pixel accumulates one unit per fragment that rasterized over it — the fragments a depth test would reject included, because they cost the same shading. A full-screen pass then reads the count and writes one flat band colour per layer count.

Bands: black 0, blue 1, cyan 2, green 3, yellow 4, orange 5-6, red 7-9, white 10+.

Both passes carry the view's debug channel, so neither draws for a camera that is not a capture of pass = "overdraw". The feature is created on demand by @builtin::systems.renderDiagnostics.renderDiagnostics.