[stuartparmenter]

Objective

When an Image changes but keeps the same texture descriptor, GpuImage reuses the existing texture and writes the new data with a single write_texture call that assumes the data holds only mip 0. For images with more than one mip level:

• Mip levels 1 and up keep the data from the first upload.
• Array layers after the first are written from the wrong part of the data, because each layer's data also contains its mip levels.

Solution

Write each mip level and array layer separately, using the same data layout as create_texture_with_data, which handles the first upload. If the data is shorter than the texture needs, write the levels that fit and log a warning.

Testing

• Talin tested this change and confirmed it works.
• Rendered comparison images (see below).

Showcase

A 2-layer image with 2 mip levels is uploaded gray, then updated with a different color for each layer and mip level. Each column shows one layer and mip level. Before this PR, mip 1 keeps its old gray data, and layer 1 picks up a row of layer 0's mip 1 data. The reference row shows the same data uploaded to a new texture.

AI Disclosure

This was AI assisted w/ Opus 5.5. Talin reported on Discord that uploading new data to an existing image didn't replace all of it. I used Opus to look at whether the fix belonged in asset processing or in the image upload, and we settled on the upload, since there's nothing to re-process at that point and the bug is that the new data was just being ignored. I also used Opus to build a sample app and test that render the before and after images. I then reviewed it, cleaned it up, had Talin test it, and opened PR.