Boosting brightness on fullscreen

[marcoscaceres]

A use case that seems to come up over on the Screen Wake Lock spec is people wanting to boost the brightness when they go fullscreen. There are some cases that don't need full screen, but need a wake lock (e.g., scanning a QR code, you want maximum brightness for contrast w3c/screen-wake-lock#129) - but there are several cases where folks want to go full screen, and boost the brightness also (e.g., concert app).

[rakuco]

Somewhat related: #185