Add texture category: Disable alpha for tagged (legacy) textures

[xoxor4d]

PR for a community request:

Uncle Burrito: Hey mark, some newer ish games that run with remix often use the alpha channel to store non-trasnparency data such as specular maps. Remix interprets that as transparent and makes materials that are supposed to be solid, tranparent. As far as I know, there's no such option in remix to ignore transparency on a per texture basis. Any workarounds?

Besides adding a new texture category, it also adds a checkbox Ignore Texture Alpha Channel to
Material Options (optional) -> Legacy Material Defaults

Usage footage (Showcase discord server): https://discord.com/channels/1028444667789967381/1028444668918235197/1262378351457341522

Thanks to Mark for the excellent guidance once again!

[MarkEHenderson]

Tracked as REMIX-3366

[MarkEHenderson]

This has been merged as 2ef6352

With some additional changes I wound up adding here:
5876feb