Updated the GLES shaders and rpi-specific cmake files

[vanfanel]

I improved the GLES scanlines shader so it does not show patterns on different screen resolutions. Also, changed gcc default flags to Rpi3, since having them as Rpi1 defaults had no sense (Cannonball is not fullspeed on a Pi1, and the Pi2 is using the same SOC as the Pi2 nowadays).

[djyt]

Sorry for the delay on merging this - have changed my e-mail notification settings! :)