When the destination rectangle didn't start at (0,0) or cover the whole
display the Dev.Draw function would continue to update the display in
full. Obviously that didn't result in a good output.
This change updates the drawing logic to only update the affected area
(aligned to whole bytes on the horizontal axis) and limits the amount of
transferred data to the minimum needed to cover the destination
rectangle.
The calculation of the various offsets as well as the function sending
the image data are written to support unittesting.
Signed-off-by: Michael Hanselmann <public@hansmi.ch>
Michael Hanselmann