windows: fix mouse-button-down encoding

Previously, pressing the left mouse button incorrectly reported that
X11MOUSE_BUTTON3 (right button) was pressed, and right or middle
buttons were incorrectly reported as modifiers (ctrl/shift/etc), as
they ended up outside the 2-bits which x11 mouse uses for button id.

Now left/middle/right are reported as x11 BUTTON 1/2/3, respectively.

This doesn't have any visible impact, because currently there's only
mouse-button-up binding, and less doesn't care which button it was.

However, the next commit will add right-mouse-button binding, so
ensure the windows mouse handling doesn't become broken.

screen.c

@@ -2861,14 +2861,19 @@ static int win32_mouse_event(XINPUT_RECORD *xip)
 		return (FALSE);
 
 	/* Generate an X11 mouse sequence from the mouse event. */
+	/* TODO: switch to the 1006 protocol to allow specific-button-up reports */
 	switch (xip->ir.Event.MouseEvent.dwEventFlags)
 	{
 	case 0: /* press or release */
 		if (xip->ir.Event.MouseEvent.dwButtonState == 0)
 			b = X11MOUSE_OFFSET + X11MOUSE_BUTTON_REL;
-		else if (!(xip->ir.Event.MouseEvent.dwButtonState & (FROM_LEFT_3RD_BUTTON_PRESSED | FROM_LEFT_4TH_BUTTON_PRESSED)))
-			b = X11MOUSE_OFFSET + X11MOUSE_BUTTON1 + ((int)xip->ir.Event.MouseEvent.dwButtonState << 1);
-		else
+		else if (xip->ir.Event.MouseEvent.dwButtonState == 1)  /* leftmost */
+			b = X11MOUSE_OFFSET + X11MOUSE_BUTTON1;
+		else if (xip->ir.Event.MouseEvent.dwButtonState == 2)  /* rightmost */
+			b = X11MOUSE_OFFSET + X11MOUSE_BUTTON3;
+		else if (xip->ir.Event.MouseEvent.dwButtonState == 4)  /* middle ("next-to-leftmost") */
+			b = X11MOUSE_OFFSET + X11MOUSE_BUTTON2;
+		else  /* don't bother to figure out what changed */
 			return (FALSE);
 		break;
 	case MOUSE_WHEELED: