Fix clipping issue with bold special characters

fc0abfae113f185cd4ccfebab2a3e0168b5224d3 re-introduced the clipping bug https://github.com/JetBrains/jediterm/issues/60, where characters are drawn outside their cell bounds.

Clipping computation based on `gfx.getFontMetrics(font).charsWidth()` is kind of a no-op, because it is precisely the width the text will want to take. So it won't avoid too large text rendering.

I've fixed and moved the clipping logic in `drawChars()`.