- fixed allocation bugs detected by valgrind
- fixed bugs detected by 'tstbackend -r 1 -l 1'
- allow 'genesys' as default value for backend opening
- made OPT_COLOR_FILTER effectively work for GL646 scanners
- split sane_control_options() to make it more readable
options for enabling/disabling horizontal interpolation at high
resolutions and selection of color filter in grayscale and
lineart modes.
* backend/genesys_gl841.c: implemented color filter selection.