Blending modes in materials/particles

[ada-tv]

• Additive blending: Useful for particles, can be used for a limited form of order-independent transparency if negative colors are allowed
• Multiplicative blending: Handy for tinted glass or fog effects
• Premultiplied alpha blending: The correct way I should have implemented Fix transparent web entity alpha #1819 and Un-premultiply overlay desktop texture #1895, had I known this was an option earlier. Also handy for some particles, and will make it possible to correctly render materials using web entity texture sources
• Maybe dedicated opaque and alpha clip/hash modes? In this case we'd keep the existing behavior with the "alpha" blending mode being opaque if the color has an alpha of 1. I think these might need shader variants for alpha discarding, while the other ones don't need any special shaders and are done in fixed-function hardware after the fragment shader.

We could potentially expose the blending operators directly for maximum control. Most engines don't bother and instead only expose the common useful ones as presets, since they're also a lot more intuitive than an operator chain.