Fluid: Optimization for smoke simulation (multigrid)

The solver will now automatically detect static scenes (no moving obstacles) and use a slightly faster pressure solve in those cases.

extern/mantaflow/preprocessed/plugin/pressure.cpp

@@ -1171,6 +1171,11 @@ void solvePressureSystem(Grid<Real> &rhs,
     maxIter = 100;
 
     pmg = gMapMG[parent];
+    // Release MG from previous step if present (e.g. if previous solve was with MGStatic)
+    if (pmg && preconditioner == PcMGDynamic) {
+      releaseMG(parent);
+      pmg = nullptr;
+    }
     if (!pmg) {
       pmg = new GridMg(pressure.getSize());
       gMapMG[parent] = pmg;