BoUT2: The ultimate DX9Settings.ini for Const lovers

- 15 key bindings! (not all useful for this game, but I kind of want to be able
  to grab this as a template when hacking other shaders).

- Number key row sets Const3 between 0.0 and 1.0 in 0.1 increments. `~ is 0.0,
  1-9 sets it in 0.1 increments and 0 sets it to 1.0

- U, I, O and P toggles Const1, 2, 3 and 4 respectively between 0.0 and 1.0

- Shadows are on Const1, so U now toggles shadows
BoUT2: Remove mirror vertex shader

I didn't manage to fix this, so no point tracking it unless I make a
breakthrough.