CHARSET does not accurately display correct characters

[Tunahan-Uysal]

When creating an image with the charset of 'abcdefghijklmnopqrstuvwxyz' and running it using an image of a gradient going from black to white, the characters present are heavily limited and only show half of the alphabet, only up to the character 'i'.

with the command ./main -p ./gradient-black-to-gray.png -s 50 20 -v -c 'abcdefghijklmnopqrstuvwxyz'

aaaaaaaaaaaaaabbbbccccddddeeeefffffgggghhhhhiiiiii
aaaaaaaaaaaaaabbbbccccddddeeeefffffgggghhhhhiiiiii
aaaaaaaaaaaaaabbbbccccddddeeeefffffgggghhhhhiiiiii
aaaaaaaaaaaaaabbbbccccddddeeeefffffgggghhhhhiiiiii
aaaaaaaaaaaaabbbbbccccddddeeeefffffgggghhhhhiiiiii
aaaaaaaaaaaaabbbbbccccddddeeeefffffgggghhhhhiiiiii
aaaaaaaaaaaaabbbbbccccddddeeeefffffgggghhhhiiiiiii
aaaaaaaaaaaaabbbbbccccddddeeeefffffgggghhhhiiiiiii
aaaaaaaaaaaaabbbbcccccddddeeeefffffgggghhhhiiiiiii
aaaaaaaaaaaaabbbbcccccddddeeeefffffgggghhhhiiiiiii
aaaaaaaaaaaaabbbbcccccddddeeeefffffgggghhhhiiiiiii
aaaaaaaaaaaaabbbbcccccddddeeeeffffggggghhhhiiiiiii
aaaaaaaaaaaaabbbbcccccddddeeeeffffggggghhhhiiiiiii
aaaaaaaaaaaaabbbbcccccddddeeeeffffggggghhhhiiiiiii
aaaaaaaaaaaaabbbbcccccddddeeeeffffgggghhhhhiiiiiii
aaaaaaaaaaaaabbbbcccccdddeeeeeffffgggghhhhhiiiiiii
aaaaaaaaaaaaabbbbcccccdddeeeeeffffgggghhhhhiiiiiii
aaaaaaaaaaaaabbbbcccccdddeeeeeffffgggghhhhhiiiiiii
aaaaaaaaaaaaabbbbccccddddeeeefffffgggghhhhhiiiiiii
aaaaaaaaaaaaabbbbccccddddeeeefffffgggghhhhhiiiiiii

changing the size with '-s' does not affect the result

this issue also occurs when trying to use an emoji charset, creating various missing characters and some not even present before