Add ability to get screenshot

[crertel]

It'd be great if the driver supported some debug command to dump its current window contents to a PNG or similar file.

[boydm]

This is good to add, but it won't be in 0.11

[axelson]

This would be very helpful for https://github.com/pappersverk/scenic_driver_inky/ as well, since that currently uses the rpi framebuffer to grab a "screenshot"

[axelson]

This might also be helpful for generating images/photos to use for visual regression testing with ex_unit