Update function for fallback resolution, mimick behavior of tesseract

[timvisee]

• [x] Fallback for all DPI values outside the allowed range, not just 0.
• [x] Use DPI range constants through bindings, depends on https://github.com/antimatter15/tesseract-rs/issues/8
• [x] ~Only use fallback DPI if not explicitly specified~

This PR updates the behavior of the set_fallback_source_resolution function. It shouldn't only fallback if the DPI is 0, and it should only fallback if the user didn't explicitly specify a DPI.

More information: https://github.com/houqp/leptess/issues/6#issuecomment-541464674

Fixes #6