Radical simplification of the wind effect on hair, since the previous

approach does not work very well.

Using a cross section estimate still causes large oscillations due to
varying hair force based on angles. It also requires a sensible hair
thickness value (particle radius) which is difficult to control and
visualize at this point.

The new model is based purely on per-vertex forces, which seems to be
much more stable. It's also somewhat justified by the fact that each
hair vertex represents a certain mass.

Conflicts:
	source/blender/physics/intern/BPH_mass_spring.cpp

source/blender/physics/intern/implicit_blender.c

@@ -1493,11 +1493,22 @@ void BPH_mass_spring_force_edge_wind(Implicit_Data *data, int v1, int v2, float
 	add_v3_v3(data->F[v1], f);
 	
 	world_to_root_v3(data, v2, win, winvec[v2]);
-	/* use -length to invert edge direction */
 	edge_wind_vertex(dir, length, radius2, win, f, dfdx, dfdv);
 	add_v3_v3(data->F[v2], f);
 }
 
+void BPH_mass_spring_force_vertex_wind(Implicit_Data *data, int v, float UNUSED(radius), const float (*winvec)[3])
+{
+	const float density = 0.01f; /* XXX arbitrary value, corresponds to effect of air density */
+	
+	float wind[3];
+	float f[3];
+	
+	world_to_root_v3(data, v, wind, winvec[v]);
+	mul_v3_v3fl(f, wind, density);
+	add_v3_v3(data->F[v], f);
+}
+
 BLI_INLINE void dfdx_spring(float to[3][3], const float dir[3], float length, float L, float k)
 {
 	// dir is unit length direction, rest is spring's restlength, k is spring constant.