Composition of the fluid and the game (depth testing, blending)

[yogwoggf]

Scope

• Pass our depth buffer into D3D9, maybe via EGSM, to composite our fluids with the game world.
• Write a D3D9 shader to depth test and discard pixels of the fluid texture that fail the depth test

Acceptance criteria

• The fluid is depth tested and cannot be seen through objects closer than the fluid is to the camera.
• The rendering isn't slowed down

[yogwoggf]

So, the acceptance criteria hasn't been met, but the latest commit, 85fe56e, introduced the groundwork for this. This is native, so we're not going to have to use EGSM. Hopefully.