Attempt to add support for 9-slice

[JohnEmhoff]

Hello! I'm trying to add support for nuklear's 9-slice rendering to love-nuklear. If you aren't familiar, it's a way to specify a texture for use as a button / window / whatever in a manner that avoids scaling artifacts at the corners and edges.

I've done a (quick-and-dirty for now) modification of nk_love_style_push_item to additionally accept 9-slice args ({image, quad, top, left, bottom, right}) in addition to its current support for colors and images. Mechanically things are working, but it renders very oddly (see below). There are enough unknowns that I'm having a hard time debugging:

• The lua API is fairly new to me (but I think I have this right)
• nuklear itself is new to me and the docs aren't great, although after studying the source and the 9-slice PR in its repo I think I have the arguments correct (see the comment in skin.lua)
• I could be misusing some of nuklear-love's abstractions

I've modified the "skin" example as a test -- I'm trying to render the normal button using a 9-slice. What I get is below; note the odd tiling effect in the button rendering.

Anywhoo, I'm going to continue to debug but I thought I'd throw this up in case more experienced eyes can see the thing that I'm doing wrong.

[JohnEmhoff]

Ah, the issue seems to be that when drawing a sub-image love-nuklear does not use the sub-rectangle fields inside nk_image and instead calls getViewport on the source image. That's why it's always drawing the entire texture for each of the patches -- it does not know to draw a sub-sub image. I'm sure there's a simple solution that I'll stumble on eventually.