Option to cap a texture export to a maximum size

[dpogue]

Thinking about this in the context of the work to get book pages consistent, and it would be nice to have a way to say "Use this 1024×1024 texture, but export it at a maximum size of 256×256" so that artists don't have to resize textures manually.

Based on #1, #2, etc. suffixes in Cyan-exported textures, I'm pretty sure Max has a way to do this, but I don't know how that works.

In our case, we'd need to decide if that happens on the image level, or the layer level, and how we'd differentiate the same texture being exported with different sizes.

[Hoikas]

We probably don't want the same texture being exported at multiple sizes. I feel like the only time you'd want that is if you're doing something really esoteric with per-page textures and pages being paged in/out to manage a texture budget. At this point, modern hardware has enough VRAM that the engine itself will probably hit file size limitations before we run out of VRAM, so I don't think we need to bother with that kind of situation. I do think that being able to use a huge texture in Blender and scale it down on export is useful, however.