Variable Font._textSizeBuffer

Buffer to reuse while measuring strings to avoid GC

class Font
{
  // ...
  int[] _textSizeBuffer;
  // ...
}

This array store character widths cumulatively. For example, after measure of monospaced 10-pixel-width font line "abc def" textSizeBuffer should contain something like: [10, 20, 30, 40, 50, 60, 70]