WIP: Implement Goldsrc render modes

[eira-fransham]

Goldsrc handles some properties on a per-entity basis that in Bevy are handled in the material properties. This PR has an initial implementation of bridging that gap. Currently the rendermode/renderamt keys and the meaning of their values are hard-coded in bsp::loader::scene (and .map files don't have any handling for it, even though they should probably allow it too), but it shouldn't be too tough to allow this to be configurable.

I originally just implemented this because I wanted to get a nice screenshot of one of the Anomalous Materials levels but honestly being able to control material properties per-entity would probably be a useful tool for general level design if it was made configurable.

I think if this was configurable, you could add a "goldsrc_material_property_hook" (or some similar name) that implements HL1-style materials, so if people want HL1-compatible behaviour they have an implementation they can plug in, but the default should probably still be Q1-compatible. Maybe there could be builder methods like BspConfig::default_quake()/BspConfig::default_quake2()/BspConfig::default_half_life() that make a best-effort attempt at using the right hooks etc.

Here's some translucent glass with this technique:

There's something odd going on with the implementation of the Goldsrc render modes but I think the general technique for modifying textures makes sense overall.