Some bad distributions (Debian) apparently are not good enough to
have a Pillow version from this decade packaged.
Therefore, we need to do it the painful way of prefixing our
symbols and refactoring everything to use them.
A new header file called "utils.h" has been added for this purpose,
and it is included in "overviewer.h".
The following macros have been prefixed with "OV_":
- MIN
- MAX
- CLAMP
- BLEND
- MULDIV255
Additionally, the C extension version was bumped to 56 because 55
was reverted back to 54.
Today in "why did we ever do this?", we discover we've vendored
code from PIL/Pillow, but PIL/Pillow moved some macro definitions
around, so now our vendored code conflicted with their macros that
they included in the headers we're including.
The solution is to throw out our vendored macros and update the
semantics in our vendored Draw.c. I'm not sure why we vendored it,
but we seemingly did remove some stuff from it to avoid having to
pull in all of PIL/Pillow.
