Blender Git Loki

Gpencil Offset Modifier - Add randomize offset options

This patch adds the Randomize options that exist in the Array modifier to the offset modifier.

Currently the patch uses
```
BLI_findindex(&gpf->strokes, gps);
```
to get the index of the current stroke for making each stroke a different seed value. This is how the noise modifier also gets the stroke seed value and it is noted there as well that this method is slow, and should be fixed in the future with another method of getting the stroke index.
Other methods were explored such as using the total number of points of the stroke, but that makes the randomize options incompatible with other modifiers before it such as Multiple Strokes, Array, Build, and Simplify.

{F9591325}

Differential Revision: https://developer.blender.org/D10171