Fluid: Added new viscosity solver

Mainly updated the Mantaflow version. It includes the new viscosity solver plugin based on the method from 'Accurate Viscous Free Surfaces for Buckling, Coiling, and Rotating Liquids' (Batty & Bridson).

In the UI, this update adds a new 'Viscosity' section to the fluid modifier UI (liquid domains only). For now, there is a single 'strength' value to control the viscosity of liquids.

release/scripts/startup/bl_ui/properties_physics_fluid.py

@@ -1006,6 +1006,46 @@ class PHYSICS_PT_particles(PhysicButtonsPanel, Panel):
                 split.operator("fluid.free_particles", text="Free Particles")
 
 
+class PHYSICS_PT_viscosity(PhysicButtonsPanel, Panel):
+    bl_label = "Viscosity"
+    bl_parent_id = 'PHYSICS_PT_liquid'
+    bl_options = {'DEFAULT_CLOSED'}
+    COMPAT_ENGINES = {'BLENDER_RENDER', 'BLENDER_EEVEE', 'BLENDER_WORKBENCH'}
+
+    @classmethod
+    def poll(cls, context):
+        # Fluid viscosity only enabled for liquids
+        if not PhysicButtonsPanel.poll_liquid_domain(context):
+            return False
+
+        return (context.engine in cls.COMPAT_ENGINES)
+
+    def draw_header(self, context):
+        md = context.fluid.domain_settings
+        domain = context.fluid.domain_settings
+        is_baking_any = domain.is_cache_baking_any
+        has_baked_any = domain.has_cache_baked_any
+        self.layout.enabled = not is_baking_any and not has_baked_any
+        self.layout.prop(md, "use_viscosity", text="")
+
+    def draw(self, context):
+        layout = self.layout
+        layout.use_property_split = True
+
+        domain = context.fluid.domain_settings
+        layout.active = domain.use_viscosity
+
+        is_baking_any = domain.is_cache_baking_any
+        has_baked_any = domain.has_cache_baked_any
+        has_baked_data = domain.has_cache_baked_data
+
+        flow = layout.grid_flow(row_major=True, columns=0, even_columns=True, even_rows=False, align=False)
+        flow.enabled = not is_baking_any and not has_baked_any and not has_baked_data
+
+        col = flow.column(align=True)
+        col.prop(domain, "viscosity_value", text="Strength")
+
+
 class PHYSICS_PT_diffusion(PhysicButtonsPanel, Panel):
     bl_label = "Diffusion"
     bl_parent_id = 'PHYSICS_PT_liquid'
@@ -1470,6 +1510,7 @@ classes = (
     PHYSICS_PT_noise,
     PHYSICS_PT_fire,
     PHYSICS_PT_liquid,
+    PHYSICS_PT_viscosity,
     PHYSICS_PT_diffusion,
     PHYSICS_PT_particles,
     PHYSICS_PT_mesh,