fix: clamp mouse position into the terminal dimension

[tauu]

When determining the Mouse Offset, it can happen that the resulting position is outside of the terminal. E.g. if a mouse is used to select text and moved outside the terminal, the reported position may be outside the terminal. Currently in these case it can happen that the text selection has e.g. a negative x position.

As it doest not make sense, that the TextSelection contains a position outside the terminal, this PR ensures that this can no longer happen. Therefore additional checks are added to getMouseOffset which clamps the position into the terminal area.

[devmil]

nit: There is an easier way of expressing the clamping

Suggested change

	x = math.min(math.max(x, 0), widget.terminal.terminalWidth - 1);
	y = math.min(math.max(y, 0), widget.terminal.terminalHeight - 1);
	x = x.clamp(0, widget.terminal.terminalWidth - 1);
	y = y.clamp(0, widget.terminal.terminalHeight - 1);