Improve texture viewer to support streamed atlases

[tccountus]

Even with brakahnes pull request applied to address the memory issue or without the fix for the addresses, the new Texture viewer is broken:

It loads when I move and somehow corrupts. I just added terrain and clicked on it. I also got the terrain texture to corrupt somehow.

After I moved a while:

[tccountus]

When I click on clear texture and then on select texture to select the grass again, then move, this happens:

[tccountus]

It's also present on each new chunk

[nxrighthere]

The preview isn’t actually corrupted. This happens because the terrain uses virtual texture atlases with streaming. This issue falls under the category of improvements rather than bugs.