term_mouse: make special treatment of x < 0 more selective.

A mouse drag which manages to reach x < 0 (via SetCapture or
equivalent) was treated as having the coordinates of (x_max, y-1).
This is intended to be useful when the mouse drag is part of ordinary
raster-ordered selection.

But we were leaving that treatment enabled even for mouse actions that
went to xterm mouse tracking mode - thanks to Markus Gans for
reporting that - and when I investigated, I realised that this isn't a
sensible transformation in _rectangular_ selection mode either. Fixed
both.

terminal.c

@@ -6088,7 +6088,18 @@ void term_mouse(Terminal *term, Mouse_Button braw, Mouse_Button bcooked,
 	    term_scroll(term, 0, +1);
     }
     if (x < 0) {
-	if (y > 0) {
+	if (y > 0 && !raw_mouse && term->seltype != RECTANGULAR) {
+            /*
+             * When we're using the mouse for normal raster-based
+             * selection, dragging off the left edge of a terminal row
+             * is treated the same as the right-hand end of the
+             * previous row, in that it's considered to identify a
+             * point _before_ the first character on row y.
+             *
+             * But if the mouse action is going to be used for
+             * anything else - rectangular selection, or xterm mouse
+             * tracking - then we disable this special treatment.
+             */
 	    x = term->cols - 1;
 	    y--;
 	} else