EMI: Fix font rendering with TinyGL.

This PR fixes the text drawing segfaults when using TinyGL. The issue was that the size of the bitmap was being calculated incorrectly, causing a buffer overrun. This adds a function to the Font class that finds the height of the bitmap version of the string. I also add the remainder of the difference between the kerned width and the bitmap font character for the last character to the string width. Finally, I added an assert to make sure that this fix is correct which could be removed if it's not needed in the main branch.
That's what it was before. I'll change it.