This caused regressions with some of our samples like vk_hellopbr:
- For the new 1x1 textures, RGB isn't accepted by Vulkan and Metal.
Currently these platforms require RGBA, although we plan on adding
reshaping functionality for the future.
- Too many texture samplers in a single shader, this causes a run time
error. This could be alleviated by creating an atlas.
- vk_hellopbr assumes that all materials have a "metallic" param.
Going forward, we plan on creating a new library that avoids MeshAssimp,
so for now let's just disable gltf_viewer.
b3dce967eb