Merge PR 'Make particles solid squares when cl_particles_quake is set to 2'

Merge PR 'Make particles solid squares when cl_particles_quake is set to 2'

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.

Improves particle function documentation.

See https://github.com/DarkPlacesEngine/darkplaces/pull/137

Closes https://gitlab.com/xonotic/darkplaces/-/issues/308

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>