Bullet 2.75 released: GPU, SPH fluids, overall improvements

[Stefan]

Bullet 2.75 is available for download now (SVN revision 1754)

Some features:

    * Smoothed Particle Hydrodynamics (SPH) fluids preview, thanks to Rama Hoetzlein for contributing this under the ZLib license! See Bullet/Extras/sph. The SPH fluids will be integrated into Bullet with two-way interaction, similar to cloth/soft body simulation.
    * GPU physics kernels preview, in preparation for Bullet 3.x. Right now mainly CPU version, you can enable CUDA version manually. The OpenCL version is available in a separate branch. See Bullet/Demos/Gpu2/3dDemo
    * 2D physics demo, with 2D and 3D object interaction, see Bullet/Demos/Box2dDemo
    * New constraints: btHinge2Constraint,btUniversalConstraint,btGeneric6DofSpringConstraint, thanks to Roman Ponomarev
    * Use ALT+Left/Middle/Right Mouse button+mouse motion for Maya-style camera navigation
    * Re-enabled split impulse constraint solver option, to avoid adding momentum due to penetration recovery (includes SIMD support)
    * Removed first argument 'localInertiaTensor, from btMultiSphereShape. Update your code by remove the first argument: it calculates its local inertia from its aabb now.
    * Increased performance by disabling motion state synchronization for static/inactive objects.
      Use btDiscreteDynamicsWorld::setSynchronizeAllMotionStates(true); for backwards compatiblity.
    * Improvements in Soft Body simulation: added tetrahedral volumetric demos, allow for per-triangle/per-tetrahedron collision (pass 0 in generateClusters), increased stability for cluster collision, improved performance of bending constraint generation.
    * Added pre-tick callback option, called at the start of each internal simulation substep.
    * Added optimized capsule-capsule collider, useful for ragdolls.
    * many bug fixes