Boundaries and color

This update includes a lot of changes to the simulation algorithm as well as some new features. I ended up restructuring the velocity solve to mirror the one found in GPU Gems, which is still based on Stam's solution, but is more clear about the pressure and temperature fields, and simplifies the gradient subtraction a bit, so the performance is better.

For the reaction coupling, I diffuse two density fields with a laplacian kernel and apply the Gray-Scott equation, then advect them. Since there is a slight amount of diffusion from bilinear interpolation in the advection step, I suspect the Gray-Scott feed and kill values might be a little off. Nonetheless it appears stable. I'm passing a texture from Touch into cuda to define obstacles. The color is coming from the velocity, with hue mapped to its polar angle.

There are a lot of fields and variables to play with now. I'll have to decide if I want to start working on robustness and composition techniques, or to move forward with adding a particle system and pushing the resolution as far as possible.