9-slice/9-patch support for skinning about nuklear

[dumblob]

I'm not any more that reluctant to this functionality - how big is that change? I'd very much prefer to make this change an addition, i.e. fully backwards compatible.

[Michael-Kelley]

It's essentially a copy of nk_image.c for 9-slice images, an additional style function (nk_style_item_9slice), relevant additions to nk_style_item_type and nk_style_item_data, and modifications to widget drawing functions to add support for 9-slice alongside image and color. It uses a separate function to draw (calls nk_draw_image for each slice) and introduces no breaking changes.

[dumblob]

Which backends did you implement? Just make a PR and let's discuss it more technically to smoothen the potential sharper edges 😉 (don't forget to bump version, make changes to src/* and then run src/paq.sh or src/paq.bat).

[wheybags]

Just a note that I would be interested in using this for https://github.com/freeablo.
Especially if there was a mode to allow repeating the edge sections instead of stretching them.

[RobLoach]

I believe 9-slice has been implemented. I haven't used it, nor have a seen a demo of it though. Feel free to re-open if you'd like some clarification on it! 🚀 Was over at #304