Batching - Protection against zero and small sized ninepatches

[lawnjelly]

Although the minimum size of ninepatches is set to the sum of the margins in normal use (through gdscript etc) it turns out that it is possible to programmatically create ninepatches that are small than this minimum - in particular zero size is used in sliders to not draw items.

This PR deals with zero sized ninepatches by not drawing anything, and has some basic protection for ninepatches smaller than the margins. Whether these occur in the wild is not clear but is put in for completeness.

Fixes @oeleo1's slider issue in #42899

Notes

• Although it is good putting these in for protection, it would be more efficient for sliders (and other items) not wishing to draw themselves, to not issue draw commands, rather than set ninepatch size to zero.
• On the other hand it may be that it makes the UI code simpler doing it like this, and this may outweigh efficiency concerns in the rendering. I haven't examined the UI code.

[akien-mga]

Thanks!