Left-handed y-up cubemap coordinates

[danchia]

Objective

• Texture cubemaps, such as used by point light shadows, inherently use left-handed y-up coordinate axes. Prior to this PR, I understand that we attempted to keep cubemaps mostly right-handed. However, the solution has sharp edges, such as requiring the sampling vector to still be negated, as well as hard-to-figure reasons for why the cubemap faces use a specific target/up axis.
• Solves the same problem as bevy_pbr: Use left-handed coordinates for point light shadow cubemaps #4242, but with a slightly different approach.

Solution

• We embrace texture cubemaps being left-handed y-up:
  • When it comes time to sample them we flip the z coordinate to transform bevy's right-handed y-up coordinates to left-handed y-up coordinates.
  • When rendering the cubemap, carefully pick the right Bevy coordinate-space target/up axis to produce the correct cubemap faces accounting for the fact that the cubemap faces are specified in the left-hand coordinate space.
  • Note that the in prior PR bevy_pbr: Use left-handed coordinates for point light shadow cubemaps #4242, we converted the entire shadow rendering pipeline for the cubemap to left-handed coordinate space. I felt that carefully mapping the cubemap faces to the right axes led to a simpler solution overall, even if the cubemap face mapping had to be done carefully.

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Changelog

• Bevy point light shadow cubemaps now use a left-handed y-up coordinate space.

Migration Guide

• When sampling from the point light shadow cubemap, use the (expected) light to fragment direction vector but negate the z coordinate. Previously, you would have used the fragment to light direction vector.

[superdump]

Simple, clean, good stuff.

[robtfm]

lgtm apart form 1 nit

[danchia]

Thanks for the reviews!