Draft: environment: Allow the core to borrow the framebuffer
This implements RETRO_ENVIRONMENT_GET_CURRENT_SOFTWARE_FRAMEBUFFER to let the core borrow the framebuffer in some cases to directly draw on it and spare us from a buffer copy.
This also give RetroFramebuffer the set/get_is_borrowed() methods and RetroPixelFormat the get_pixel_size() method.
Draft because I left a FIXME I need to sort our first, even though it's nothing super important.
Edited by Adrien Plazas