(really?) fix refresh viewport calculations

This adds more documentation about the viewport offset calculations
for refreshing rotated viewports.

The old behaviour also was (still) buggy, even after the latest round
of fixes. Now, the offsets when rotated are hopefully calculated correct.

frontend/device/screen.lua

@@ -80,23 +80,52 @@ end
 
 function Screen:refresh(refresh_type, waveform_mode, wait_for_marker, x, y, w, h)
     if self.viewport and x and y then
-        -- adapt to viewport, depending on rotation
+        --[[
+            we need to adapt the coordinates when we have a viewport.
+            this adaptation depends on the rotation:
+
+          0,0               fb.w
+            +---+---------------------------+---+
+            |   |v.y                     v.y|   |
+            |v.x|                           |vx2|
+            +---+---------------------------+---+
+            |   |           v.w             |   |
+            |   |                           |   |
+            |   |                           |   |
+            |   |v.h     (viewport)         |   |
+            |   |                           |   | fb.h
+            |   |                           |   |
+            |   |                           |   |
+            |   |                           |   |
+            +---+---------------------------+---+
+            |v.x|                           |vx2|
+            |   |vy2                     vy2|   |
+            +---+---------------------------+---+
+
+            The viewport offset v.y/v.x only applies when rotation is 0 degrees.
+            For other rotations (0,0 is in one of the other edges), we need to
+            recalculate the offsets.
+        --]]
+
+        local vx2 = self.screen_size.w - (self.viewport.x + self.viewport.w)
+        local vy2 = self.screen_size.h - (self.viewport.y + self.viewport.h)
+
         if self.cur_rotation_mode == 0 then
             -- (0,0) is at top left of screen
             x = x + self.viewport.x
             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:getWidth()-self.viewport.h)
+            x = x + vy2
             y = y + self.viewport.x
         elseif self.cur_rotation_mode == 2 then
             -- (0,0) is at bottom right of screen
-            x = x + (self.fb.bb:getWidth()-self.viewport.w)
-            y = y + (self.fb.bb:getHeight()-self.viewport.h)
+            x = x + vx2
+            y = y + vy2
         else
             -- (0,0) is at top right of screen
             x = x + self.viewport.y
-            y = y + (self.fb.bb:getHeight()-self.viewport.w)
+            y = y + vx2
         end
     end
     self.fb:refresh(refresh_type, waveform_mode, wait_for_marker, x, y, w, h)