This switches to 64-bit integers in select locations of the Image
class, so that image resolutions of 16384×16384 (used by
lightmap texture arrays) can be used properly. Values that are larger
should also work.
VRAM compression is also supported, although most VRAM-compressed
formats are limited to individual slices of 16384×16384. WebP
is limited to 16383×16383 due to format limitations.
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