Prepare for Linux(GCC) and MinGW compilation.
(Screen space fluid renderer has linking
issues with Glew on Linux/MinGW; OpenCL untested).
Hf: improve rigid volume estimation and collisions; fix density.

-Add higher resolution voxels for volume estimation.
-Use cubes instead of spheres for volume estimation.

-Compare heights instead of testing for intersection
for hfFluid-rigid interaction by default(improves stability).

-Fix to use fluid density instead of rigid body density for buoyancy,
add option to use same density as fluid for rigid bodies.
(Rigid body would always float regardless of mass
unless 'floatyness' was set.)

Also rename 'floatyness' to 'buoyancyScale'.
Add partially absorbing SphHf interaction test.
(Particles are absorbed by the heightfield when
 the number of particles exceeds a threshold.)
Sph: add tube-shaped trimesh test to soft body demo.