Use bit shifts when tracking mouse button state

I think it makes more sense to use bit shifts instead of decimals for
each button.

vncviewer/Viewport.cxx

@@ -614,37 +614,37 @@ int Viewport::handle(int event)
     // resolved in FLTK.
     mouseButtonMask &= ~(0x07);
     if (Fl::event_button1())
-        mouseButtonMask |= 1;
+        mouseButtonMask |= 1 << 0;
     if (Fl::event_button2())
-        mouseButtonMask |= 2;
+        mouseButtonMask |= 1 << 1;
     if (Fl::event_button3())
-        mouseButtonMask |= 4;
+        mouseButtonMask |= 1 << 2;
 
     switch (event) {
     case FL_PUSH:
       if (Fl::event_button() == 8)
-        mouseButtonMask |= 128;
+        mouseButtonMask |= 1 << 7;
       else if (Fl::event_button() == 9)
-        mouseButtonMask |= 256;
+        mouseButtonMask |= 1 << 8;
       break;
     case FL_RELEASE:
       if (Fl::event_button() == 8)
-          mouseButtonMask &= ~(128);
+          mouseButtonMask &= ~(1 << 7);
       else if (Fl::event_button() == 9)
-        mouseButtonMask &= ~(256);
+        mouseButtonMask &= ~(1 << 8);
       break;
     }
 
     if (event == FL_MOUSEWHEEL) {
       wheelMask = 0;
       if (Fl::event_dy() < 0)
-        wheelMask |= 8;
+        wheelMask |= 1 << 3;
       if (Fl::event_dy() > 0)
-        wheelMask |= 16;
+        wheelMask |= 1 << 4;
       if (Fl::event_dx() < 0)
-        wheelMask |= 32;
+        wheelMask |= 1 << 5;
       if (Fl::event_dx() > 0)
-        wheelMask |= 64;
+        wheelMask |= 1 << 6;
 
       // A quick press of the wheel "button", followed by a immediate
       // release below