Water rendering improvements

[paulcscharf]

8476c61 adds a pretty good start at a proper water shader.

Here's a list of things that can be improved:

• [ ] remove hardcoded surface light
  • either replace by rendering water surface to same normal map (will have to see how this looks) or by a second normal map (that'll make rendering lights a lot more expensive, but the same could be used to light smoke and similar effects!)
• [ ] consider actual geometry surface normal
  • this isn't even passed into the shader right now, by considering this, larger-scale waves would look better
• [x] consider flow
  • also not passed into the shader yet. once we do that, we'll have to smoothly transition from a regular surface to a flowing one, and back without strange artifacts
  • this might require even more texture samples, but hopefully not too many
  • also, since we project caustics onto the terrain below, we might run into issues having properly flowing caustics at shallower angles, this needs testing
  • alternatively, if flow adds foam or a generally more turbulent surface, maybe the caustics will hardly be visible and they can remain non-flowing
• [ ] sort out tile edges
  • this is a big problem with deeper drops, if the difference is too high, we gotta make a clear cut in the geometry and use a different effect/shader for the flow (or maybe.. we can just use the same shader, where flowing 'down' is the final interpolated form of regular flow, or some not-too-special case)
  • using more detailed geometry, or tesselation might be needed
• [ ] handle shallow water better
  • prevent flickering, maybe even update flow to not spread as fast and thin (add min flow?)
• [ ] check on performance of generating geometry on the GPU
  • consider geometry shader, or tesselation, or a static mesh where only the actual fluid data is injected

[paulcscharf]

Added flow rendering in f08b069.