Keep anchorPoint the same when retexturing

This allows the owner to set a custom `anchorPoint` and not have the value reset to `(0.5,0.5)` on other property changes.

The comment indicates that the resetting the texture also resets the `anchorPoint`, but I could not reproduce. If it's reproducible, though, I could record the `anchorPoint` value in a local variable before the retexture and then restore it afterwards.