Mesh batching is broken on macOS

[brianpeiris]

The mesh batching example in this repo is renders incorrectly on macOS in both Firefox and Chrome (tested on a 13-inch 2018 MacBook Pro with Intel Iris Plus Graphics 655 chipset). Specifically, most of the textures on the meshes in the atrium GLB are rendered white and the mip mapping on the animated fan is incorrect. Inspecting the atlas shows that the base texture on layer 0 is being overwritten by mip maps from other levels and layers. Although a similar bug was observed with image batching, and fixed in #19, that fix does not seem to work for the atrium model. Also similarly, reducing the layer resolution does seem to work around the issue, at least partially.

Bug repro screenshot on macOS:

Working render on Windows:

[brianpeiris]

This also repros in Spoke when the "Unlit" render mode is enabled.
It also repros on a 2018 Mac Mini, which has integrated Intel UHD 630 graphics and 1.5GB of GPU memory.