Add a cursor to make placing/breaking blocks easier

[patowen]

This PR hijacks the fog shader to also add a cursor in the center of the viewport to allow players to target where they place and break blocks.

This introduces an if statement to the fragment shader, and branching in shaders is generally bad for performance in my understanding, but given its simplicity, I believe that the shader compiler should be smart enough to optimize this in a way that acts as though such branching basically does not exist.

[Ralith]

branching in shaders is generally bad for performance in my understanding

This hasn't really been true for a while. So long as register allocation costs aren't wildly unbalanced between branches, you can be pretty free with them.