Update velocity shader

[Thendeo]

Since we can now move particules with a constant velocity vector, we can implement a velocity update shader to modify particules speed over time. A great goal is to make a N-Body simulation given gravitational law. The latter might be difficult to implement at first because N-Body simulation requires mathematical simplification along use of the GPU.

[Thendeo]

UpdateVelocity shader has been implemented, kind of duplicate from advance shader. N-Body simulation is too complicated. A first step will be to use multiple framebuffer to update both position and velocity with a basic velocity modification. Basic velocity modification : speed attenuation along velocity vector direction ?