font_conv: fix trimming ttf fonts

[tehKaiN]

Description

Fixes the algorithm for trimming empty space around ttf font glyphs. Closes #201 Closes #202

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.

[tehKaiN]

Tested by issue reporter, bugs are gone!