- initialize the index field for particles.

This won't contribute to sort order so it should be the same for all particles, which it wasn't because it was never set.

src/hwrenderer/scene/hw_sprites.cpp

@@ -1164,6 +1164,7 @@ void GLSprite::ProcessParticle (HWDrawInfo *di, particle_t *particle, sector_t *
 	gltexture=nullptr;
 	topclip = LARGE_VALUE;
 	bottomclip = -LARGE_VALUE;
+	index = 0;
 
 	// [BB] Load the texture for round or smooth particles
 	if (gl_particles_style)