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/curve.cpp

@@ -445,10 +445,10 @@ void Curve::set_bake_resolution(int p_resolution) {
 	_baked_cache_dirty = true;
 }
 
-real_t Curve::interpolate_baked(real_t offset) {
+real_t Curve::interpolate_baked(real_t offset) const {
 	if (_baked_cache_dirty) {
 		// Last-second bake if not done already
-		bake();
+		const_cast<Curve *>(this)->bake();
 	}
 
 	// Special cases if the cache is too small