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.
