Modify FluidsGl example to add solid objects

The best I’ve managed is to add an extra step where the average velocity of the fluid in the solid is calculated, then set the velocity of every cell in that area to this value.

The problem is most of the other steps DON’T treat it like a solid so if fluid pushes against a small area on one side, the force ends up pushing through the object out the other side (have an image showing this but not a good way to post it).