Flip textures and UVs

[david-lively]

Description

This PR modifies the texture and model import process to comply with Unity's U-right, V-up convention.
The glTF 2.0 spec mandates that textures are anchored at UV (0,0) and the upper-left corner of the texture, with UVs increasing to the right and down. (See https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#images .)
Unity, however, assumes that UV (0,0) corresponds to the bottom-left corner of the image. Previously, the system did not flip either the textures OR the UVs, so it mostly worked, but has caused issues with tangent generation.

In the UV overlay debug images below, the six columns on the left were loaded by Cesium, while the six on the right were loaded by Unity glTFast. These were generated using the same glTF model:

This requires changes to cesium-native in CesiumGS/cesium-native#1259 .

Before this change:

After this change:

With textures flipped:

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Testing plan

Load a tileset. Use Unity's Render Debugger to visualize the Texcoord0 vertex attribute. Observe that UV coordinates increase in the right- and up-direction, and that textures are correctly flipped to match.

[j9liu]

Thanks @david-lively! I'm still building this locally to test it out, but figured I'd post my comments now.

[j9liu]

Thanks for the changes @david-lively ! Just a few small things that I won't trouble you with; I'll batch into a commit so CI can start running.