fix bug in viewport dimension check

actually, two bugs:
- upper limit was checked with bad math.
- the rectangle wasn't properly stored to self.viewport

ffi/framebuffer.lua

@@ -119,16 +119,17 @@ end
 set a rectangle that represents the area of the screen we are working on
 --]]
 function fb:setViewport(viewport)
-    if viewport.x < 0 or viewport.x >= self.screen_size.w
-    or viewport.y < 0 or viewport.y >= self.screen_size.h
-    or viewport.w < 0 or viewport.x+viewport.w >= self.screen_size.w
-    or viewport.h < 0 or viewport.y+viewport.h >= self.screen_size.h
+    if viewport.x < 0 or viewport.x > self.screen_size.w
+    or viewport.y < 0 or viewport.y > self.screen_size.h
+    or viewport.w < 0 or viewport.x+viewport.w > self.screen_size.w
+    or viewport.h < 0 or viewport.y+viewport.h > self.screen_size.h
     then
         error("bad viewport")
     end
     self.bb = self.bb:viewport(
-        self.viewport.x, self.viewport.y,
-        self.viewport.w, self.viewport.h)
+        viewport.x, viewport.y,
+        viewport.w, viewport.h)
+    self.viewport = viewport
 end
 
 function fb:calculateRealCoordinates(x, y)