Implemented dust cloud particles that spawn at mount feet when running
on the ground, similar to the original WoW. Features:
- Brownish/tan dust particles using point sprite rendering
- Spawn rate proportional to movement speed
- Particles rise up, drift backward, and fade out naturally
- Only active when mounted, moving, and on ground (not flying)
- Smooth particle animation with size growth and alpha fade
Uses similar particle system architecture as swim effects with
GL_POINTS rendering and custom shaders for soft circular particles.
