Add Wrap::UDIM mode for proper UDIM filtering

[etheory]

Currently, the wrap modes for textures are (https://openimageio.readthedocs.io/en/latest/texturesys.html):

• Wrap::Black
• Wrap::Clamp
• Wrap::Periodic
• Wrap::Mirror
• Wrap::Default

But when using UDIM textures, what you really want, is filtering across UDIM seams, else, at low mip map levels, you get strange artifacts at UDIM edges.
So I would like to propose a new Wrap mode called Wrap::UDIM that implements such filtering.

I also propose to update maketx to support this new mode somehow (new command line argument?), such that during the mip-map generation, it also performs this cross-UDIM tile filtering also.

What do people think?

I further propose that "Wrap::Default" use "Wrap::UDIM" when passed one of the UDIM-specific path types.

If you think this is a good idea, are you OK with me going ahead and proposing an implementation and making a PR?

@lgritz? Thanks.

Does this already exist and I'm missing it?