Saving files could take ~3-4 seconds on debug builds because of new
imbuf scaling logic.
Even though debug performance usually isn't much of a consideration,
it gets in the way of development.
Since thumbnails don't require the same accuracy as the sequencer or
compositor, use a faster scaling method that uses a box-filter clamped
to integer bounds & integer math.
In practice the difference between the resulting thumbnails isn't
noticeable in my tests.
For debug build with ASAN this gives a ~25x speedup,
for release builds it gives a ~1.4x speedup which is to be
expected with a more approximate scaling method.
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