[Feature] `Content` Size Option for `ImageBoxSizeValue`

[zicklag]

Since we already have the textures sizes passed into the tesselation renderer I would think it should be easy to implement. Also, it should probably be the default setting inead of Fill, I think. Content size is what I would guess would be the default image size.

[PsichiX]

great idea! that will complement the other feature of Content variant for text box sizes :D