Fix exact characters to split in three and resize to resulting character

[skotz]

The exact_characters setting will split in half but not into thirds, so this:

Splits awkwardly down the middle of the 8:

The split characters need to be cropped to the resulting character boundaries, not keep the boundaries of the pre-split image. For example:

The p needs some space cropped off the top after the split, and the 4 needs some space cropped off the bottom.

See #32

[skotz]

pr coming which fixes this