softwrap: save and restore firstcolumn when copying text

Copying text involves first cutting it and then quickly pasting it back.
However, cutting the text may change firstcolumn if the mark is offscreen.
To keep the viewport unchanged, copy_text() has to save and restore not
just edittop, but firstcolumn as well.

src/cut.c

@@ -225,6 +225,7 @@ void do_copy_text(void)
 
     /* Remember the current viewport and cursor position. */
     ssize_t is_edittop_lineno = openfile->edittop->lineno;
+    size_t is_firstcolumn = openfile->firstcolumn;
     ssize_t is_current_lineno = openfile->current->lineno;
     size_t is_current_x = openfile->current_x;
 
@@ -239,6 +240,7 @@ void do_copy_text(void)
     if (mark_set) {
 	/* Restore the viewport and cursor position. */
 	openfile->edittop = fsfromline(is_edittop_lineno);
+	openfile->firstcolumn = is_firstcolumn;
 	openfile->current = fsfromline(is_current_lineno);
 	openfile->current_x = is_current_x;
     }