Fixes bug #356 : Cairo render: Conical gradient fails render if gradient is just to Color Stops
Fixes bug #355: Inverted feathered circle crash on Cairo render
New function to set top left and bottom right corners of the render description at the same time. In some cases set the top left only or the bottom right only can lead to a width and/or height of zero although the final tl and br are legal.
Fix bug #354: Layer Checkerboard renders wrong in Cairo mode for Tile render and Preview