fix bug in viewport offset calculation for refreshes

The calculation did not correctly calculate the offset in rotated
modes when the rotation was 90 or 270 degrees. Under these circumstances,
getWidth() and getHeight() did return rotated values for the framebuffer,
while Device.viewport did return not rotated values.

frontend/device/screen.lua

@@ -87,7 +87,7 @@ function Screen:refresh(refresh_type, waveform_mode, wait_for_marker, x, y, w, h
             y = y + self.viewport.y
         elseif self.cur_rotation_mode == 1 then
             -- (0,0) is at bottom left of screen
-            x = x + (self.fb.bb:getHeight()-self.viewport.h)
+            x = x + (self.fb.bb:getWidth()-self.viewport.h)
             y = y + self.viewport.x
         elseif self.cur_rotation_mode == 2 then
             -- (0,0) is at bottom right of screen
@@ -96,7 +96,7 @@ function Screen:refresh(refresh_type, waveform_mode, wait_for_marker, x, y, w, h
         else
             -- (0,0) is at top right of screen
             x = x + self.viewport.y
-            y = y + (self.fb.bb:getWidth()-self.viewport.w)
+            y = y + (self.fb.bb:getHeight()-self.viewport.w)
         end
     end
     self.fb:refresh(refresh_type, waveform_mode, wait_for_marker, x, y, w, h)