shadertool: Add --original to work with original shader instead of the one specified

I'm planning to make heavy use of this with --git since I always try to
commit the original in a separate commit to the modified one so I keep
doing this manually.
Dreamfall Chapters: Add light shaft shader from main menu

I've identified this as one variant of FinalInterpolation.shader from
https://github.com/robertcupisz/LightShafts

Different variants are used depending on camera angle
Dreamfall Chapters: Fix light shafts on main menu

Looking at the source code for these shaders it was clear that the
_LightPos parameter is the mono screen position of the light, so that
needs to be stereoised.

The original shader source is here:

https://github.com/robertcupisz/LightShafts