represent pixels as uint8_t

It used to be bool[8], that was dumb.
remove useless function
why the hell was that even there? blame says it's been here since the
initial commit /shrug
avoid unneccicary checks in get_split_axis
return from get_split_value. how the hell did this work before
fix bug where get_split_axis always returns R, exposing a bug in get_split_value, fix that as well
remove unused stb_image_write.h
add check for image loading error
change default max width to 80 columns
prefer darker pixels for foreground if group sizes are equal
fix get_split_axis harder this time
fix image scaling, remove max_h
implement max h/w correctly
two_tone: use the less common color as the foreground
Reduce dynamic memory allocations