Add ParticleRenderer that draws every live particle from the shared
ParticleSystem as a billboarded quad. Unit quad VBO + per-instance
(pos, size, color) VBO with glVertexAttribDivisor for one draw call
per emitter. Billboards using the camera's basis vectors so quads
always face the viewer.
Fragment shader does a procedural radial falloff (no texture pipeline
needed — raindrops / snowflakes read as soft dots). AttachLocal
emitters get re-centred on the camera each frame so the rain volume
follows the player per r12 §7.
Two-pass render splits additive from alpha-blend emitters so blend
state flips once per kind rather than per-emitter.
Wired into GameWindow.OnRender after static-mesh draw with depth
write off (particles occluded by walls but don't self-occlude).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
