Particle systems: non-quad particles

[yard]

Hi guys,

Just a wild thought: would it make sense to introduce a uniform like "particleStretch" to be able to tweak default particles (which are union quads now) to a different shape (i.e. rectangle)?

Basically, converting billboard rendering into stretched billboard rendering without scaling particle system's entity (to avoid side effects of modifying world transform chain). The change seems trivial and I am happy to submit a pull request if that's fine with your design.