When running an Android VM with GL support, Android somehow enforces
support for only touch events making it difficult to work with QEMU +
Android for devolpement / demo purposes without a touch screen.
This helper is useful for being able to emulate motion / click
events as touch ones
This is a child widget to the MksDisplay which contains just the paintable
region of the remote screen. Doing so allows us to avoid having to handle
crossing events carefully for things like cursors.
Instead, now the MksDisplay calculates the area for the picture and
allocates it there. The MksDisplayPicture will always snapshot the
paintable using the full area it has been allocated.
Event processing can also largely move into the Picture widget which
simplifies bounds checking.
