Small roundoff errors during UV editing can sometimes occur, most likely
due to so-called "catastrophic cancellation".
Here we set a tolerance around zero when using Constrain-To-Bounds and UV Scaling.
The tolerance is set at one quarter of a texel, on a 65536 x 65536 texture.
TODO: If this fix holds, we should formalize the tolerance into the UV editing
subsystem, perhaps as a helper function, and investigate where else it needs
to be applied.
Differential Revision: https://developer.blender.org/D16702
0079460dc7