Text viewport isn't always accurate

[redstarcoder]

Sometimes the cursor's iOffset / iEdge are wrong, causing text to get cut off in certain situations (typically using big/small characters with a non-monospaced font).

[redstarcoder]

This might be able to be solved using FillWithin from redstarcoder/draw2dglkit and always drawing a text string bigger than the box, and setting the boundaries to the textbox boundaries. Then iOffset / iEdge would likely be measured in pixels instead of char count.