Allocate float buffer outside of image buffer,
so work-in-progress color space conversion doesn't
interfere with other parts of blender.
Covers most of cases -- since image buffer wouldn't
have partially-update float buffer all the rest
areas would be happy.
However, if there're places which updates float
buffer from byte buffer, it's still possible
some WIP color space conversion is displayed on
the screen.
But what a heck someone will do such a crappy
conversion anyway!
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