Add mount dust particle effects

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.

include/rendering/renderer.hpp

@@ -27,6 +27,7 @@ class Clouds;
 class LensFlare;
 class Weather;
 class SwimEffects;
+class MountDust;
 class CharacterRenderer;
 class WMORenderer;
 class M2Renderer;
@@ -165,6 +166,7 @@ private:
     std::unique_ptr<LensFlare> lensFlare;
     std::unique_ptr<Weather> weather;
     std::unique_ptr<SwimEffects> swimEffects;
+    std::unique_ptr<MountDust> mountDust;
     std::unique_ptr<CharacterRenderer> characterRenderer;
     std::unique_ptr<WMORenderer> wmoRenderer;
     std::unique_ptr<M2Renderer> m2Renderer;