request: Add a configuration option to adjust the speed of shader animations

[Talljoe]

I find one of the most common things I do when trying out a shader (e.g., from Shadertoys) in Ghostty is find all instances of iTime and scale them down so that the animation is far less distracting. It would be nice if I can set a configuration option to have Ghostty adjust the timescale for me. For example:

custom-shader-timescale = 10 # every "second" takes 10 seconds (1/10th speed)

This will make shaders more accessible to the non-technical user that wants to copy and paste from the internet. It will also make it easier for those that are comfortable modifying shaders to fine tune the speed of animations to their personal preference without a lot of effort.

I've built a proof of concept (linux only, no validation for <= 0, ymmv). I've optimized for slowing things down and using whole numbers for that. So rather than 0.1667 you can do 6 to get 1/6th speed. Alternatively, we can make negative numbers "slower" and positive numbers "faster." If accepted, I can drive the Linux implementation across the line but would need assistance with MacOS as I don't have (easy dev) access to one.

Extra credit: If we want to get fancy, a timescale for every custom shader:

custom-shader = "shaders/starfield.glsl?timescale=10"
custom-shader = "shaders/bettercrt.glsl?timescale=2"