rt: improve materials assembly pipeline

[w23]

Current RT material system is pretty ad-hoc with a lot of hacks and workarounds done quickly as a side thing while focusing on other parts of the renderer. It's time to make it right.

• Collect all material data into a single Material structure to be passed into shaders.
• Explicitly pass material mode: Opaque, OpaqueWithAlphaTest, Translucent (with refraction), BlendAdd, BlendMix, Skybox, Glow. Related issues: Broken rendermode "color" #310, RTX: Incorrect alpha on map c1a4d #290, Render modes for transparency #460, rtx: translucency #173
• Assign material mode based on render type. This is a first approximation, it's highly likely that the end rule and exceptions here are more complex. See materials: specify replacement textures based on rendermode #213, materials: new format #470
• Hit order independent BlendAdd+BlendMix processing.