(Libretro) Improvements

The first commit was necessary to get it to even compile (testing on Linux right now).

Going to finally get it ready for most of the libretro targets now - seems we still have a long way to go - looked in the os.cpp implementation and a lot of it still seems to be stubs.

I added RetroPad support (both D-pad and analog sticks function as mouse emulation) and I added the two mouse buttons (Mouse Button 1 to B and Mouse Button 2 to A). Start brings up the ScummVM menu.

It also sets the color format to RGB565 by default - this should help performance on Android and other lower-specced systems.