Previously, we would show the render-to-texture with its original
dimensions. While useful, this skewed the preview coordinates and was
sometimes confusing. Additionally, depth texturing didn't preview.
This pads and subsets the texture preview so it's the right size.
This means a conditional breakpoint can now use, for example,
vtype.through == 1 to check the through mode flag.
No constants, but this makes it easier to get right. Would be nice to
have an autocomplete of the field names, of course...
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