background-image-repeat is not implemented

[tfabretti]

As the title says, background-image-repeat is not implemented on Heaps. I coded a way to make it happen and will do a PR but I also open this issue to talk about it in case I did something wrong :D

As a bonus, I also added a way to make a texture repeat when using 9-slicing. To avoid strange artefacts, though, like having a visible cut between a side and the inner texture being only partially drawn, I scaled down the repeated parts to make sure only a round number of them is used.

PR incoming !