Make particles solid squares when cl_particles_quake is set to 2

[hemebond]

This creates a new function, CL_NewQuakeParticle, that is used instead of CL_NewParticle when cl_particles_quake is enabled. The function calls CL_NewParticle with a some hardcoded defaults to make the particle texture a disc, or solid square, based on the value of cl_particles_quake.

Possibly also resolves https://gitlab.com/xonotic/darkplaces/-/issues/308 by re-implementing the colour ramp mechanic for explosion particles.

[bones-was-here]

This one adds a few minor warns that should be easy to fix, we currently have no warns (at least on linux with gcc 13.2 or clang 14) so if you see one it's probably yours :) gcc seems more rigorous than clang when it comes to catching issues. Also CONTRIBUTING.md style please.

[bones-was-here]

I have a feeling that if we remove the current behaviour of cl_particles_quake 1 entirely, someone will complain.
I suggest adding this as a second mode of the cvar, eg:
cvar_t cl_particles_quake = {CF_CLIENT | CF_ARCHIVE, "cl_particles_quake", "0", "1 makes particle effects look mostly like the ones in GLQuake, 2 makes them look like the ones in software rendered Quake."};

Edit: I haven't looked at what other engines are doing here, or if there is one that offers 3 particle styles.

[hemebond]

Edit: I haven't looked at what other engines are doing here, or if there is one that offers 3 particle styles.

QuakeSpasm has OpenGL disc particles, and Ironwail allows the option of OpenGL discs or Quake squares.