[orx-shade-styles] Make it possible to apply gradients to a stroke

[hamoid]

Motivation

Would it be good to be able to specify if the shadeStyle applies to x_fill or x_stroke?

https://github.com/openrndr/orx/blob/master/orx-shade-styles/src/commonMain/kotlin/LinearGradient.kt#L78

Help make it happen!

I am willing to submit a PR to implement this change, but would need some guidance.

[hamoid]

A hacky way of doing it from user code:

val img = loadImage("data/images/cheeta.jpg")
val ss = imageFill { image = img }
ss.fragmentTransform = ss.fragmentTransform?.replace("x_fill", "x_stroke")

Basically, use any existing shade style, then make it work for stroke instead of fill by replacing a keyword in the GLSL shader which is just simply text.