Implement Particle Trails

-Enable the trails and set the length in seconds
-Provide a mesh with a skeleton and a skin
-Or, alternatively use one of the built-in TubeTrailMesh/RibbonTrailMesh
-Works deterministically
-Fixed particle collisions (were broken)
-Not working in 2D yet (that will happen next)

scene/resources/material.h

@@ -235,6 +235,7 @@ public:
 		FLAG_USE_TEXTURE_REPEAT,
 		FLAG_INVERT_HEIGHTMAP,
 		FLAG_SUBSURFACE_MODE_SKIN,
+		FLAG_PARTICLE_TRAILS_MODE,
 		FLAG_MAX
 	};
 
@@ -305,16 +306,15 @@ private:
 		uint64_t roughness_channel : get_num_bits(TEXTURE_CHANNEL_MAX - 1);
 		uint64_t emission_op : get_num_bits(EMISSION_OP_MAX - 1);
 		uint64_t distance_fade : get_num_bits(DISTANCE_FADE_MAX - 1);
-
-		// flag bitfield
-		uint64_t feature_mask : FEATURE_MAX - 1;
-		uint64_t flags : FLAG_MAX - 1;
-
 		// booleans
 		uint64_t deep_parallax : 1;
 		uint64_t grow : 1;
 		uint64_t proximity_fade : 1;
 
+		// flag bitfield
+		uint32_t feature_mask;
+		uint32_t flags;
+
 		MaterialKey() {
 			memset(this, 0, sizeof(MaterialKey));
 		}