fix: correctly encode mouse buttons >= 8 in X10 and SGR mode

These are typically mapped in X11 to the side-buttons (backward/forwards) on
the mouse. A comparison of the button numbers in SGR mode (first field):

st old:
0 1 2 64 65 66 67 68 69 70

st new (it is the same as xterm now):
0 1 2 64 65 66 67 128 129 130

A script to test and reproduce it, first argument is "h" (on) or "l" (off):

	#!/bin/sh
	printf '\x1b[?1000%s\x1b[?1006%s' "$1" "$1"

	for n in 1 2 3 4 5 6 7 8 9 10; do
		printf 'button %d\n' "$n"
		xdotool click "$n"
		printf '\n\n'
	done

x.c

@@ -387,7 +387,9 @@ mousereport(XEvent *e)
 			button = 3;
 		} else {
 			button -= Button1;
-			if (button >= 3)
+			if (button >= 7)
+				button += 128 - 7;
+			else if (button >= 3)
 				button += 64 - 3;
 		}
 		if (e->xbutton.type == ButtonPress) {