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.