Added SimpleAsyncImageDisplay.getExecutor
Adjusted SimpleAsyncImageDisplay.getImagePoint not to throw exception if the transformation is not invertible.
Fixed multiple invokes in GuiTestUtils.
Revert "AffineImageTransformer.getTransformationMatrix rotates clockwise as defined by BasicImageTransformations.". Actually, I didn't take into account that the coordinate system of images are upside down.

This reverts commit a265e2e7c2f92d42f7d3ab601c4fb2bde94bc943.
Adjusted AffineImageTransformerTest to the changes in the previous commit.
Added a little clarification to AffineImageTransformer.
Adjusted the offset calculation of AffineImageTransformer.
Adjusted the specification of SimpleAsyncImageDisplay.getImagePoint
Added tests for SimpleAsyncImageDisplay.
AsyncImageDisplay now notices the background change.