support for alpha-textures in gltf-loader

we should discuss this further, but probably something like:
- materials should have alpha information, so render pass can query this information and adjust rendering accordingly
- perhaps an `alpha rendering config` helper that provides simpler interface on common alpha blending and alpha2coverage setups?

Start by reviewing the gltf-loader, material handling, and render pass references in the issue. Clarify how materials should expose alpha information and whether an alpha rendering config helper is required; the work is complete only when the intended alpha blending and alpha2coverage behavior is specified and implemented.

Written by the indexing model from the issue text.