The Epson ES-50 (and friends) supports a larger scanning area when
used at its lowest DPI setting. This adds support for switching to
the larger reported scan area when the DPI setting is at its lowest
value.
The scanner does not directly report the DPI to scan area relationship,
so this code is based on some educated guesses and experimentation.
It's been tested to work on the ES-50 with both scanimage and xsane.
Hopefully it does not interfere with any other scanners using this
backend.
Various parts of SANE introduced their own compatibility macros for
old compilers that don't support __func__. Most of these definitions
are identical or have minor differences. This patch replaces them
with a single instance in the central header file for backends.