Onyx Boox T68 Lynx: Full support for screen dithering. Workaround for
the spurious A2 mode switch problem.

Device now switches to dithering,inverted mode; which is reliable. As a
consequence, the screen has to be inverted, implemented in TopCanvas
just before buffer flipping.
Merge remote-tracking branch 'origin/master'

Conflicts:
	src/Screen/OpenGL/Canvas.cpp