shadertool.py: --disable-texcoord, --adjust-multiply, --condition

- Can disable a specific output texcoord from the VS
- Can multiply the stereo adjustment factor (hint: close an eye while toggling
  a fix - if the broken effect moved to the other side of the goal, try 0.5)
- Can add a conditional for the disable/adjustments - e.g. specify c200.z for
  Const3 (if DefVSConst1=200)