Need metadata extension or some other solution for NV corner-sampled images

NVIDIA has an extension for corner-sampled images, which changes the rounding behavior (round-up instead of round-down) and minimum extents requirements (2 instead of 1) for image mipmaps. Since this isn't a new format or supercompression scheme, it would require some other kind of metadata signaling.

I'm guessing this needs a vendor extension, otherwise there is no way to store a mipmapped KTX for this type of image unless it has PO2 dimensions and the mip chain ends at whatever dimension reaches extents 2 first.

The issue names no file or test; start by reading the KTX specification source and the NVIDIA corner-sampled image extension requirements described in the report. Done means reaching a decided, documented way to signal the alternate mipmap rounding and minimum-extents rules.

Written by the indexing model from the issue text.