Add experimental way to make max_swapchain=2 work Rpi with GLES on dispmanx

[vanfanel]

On the Raspberry Pi, when using the propietary graphics stack, double buffer (low input lag) was only possible when using the plain dispmanx graphics backend. So the max_swapchain=2 option was ignored on this configuration.
With this small patch, eglSwapBuffers() is supposed to wait for vsync and NOT triple-buffer when max_swapchain=2. It was found out thanks to popcornmix on this thread:
raspberrypi/userland#432

[kivutar]

Thank you!

[vanfanel]

@kivutar : I need someone with the proper equipment to test this. Do you know who could do it?

[dankcushions]

@Brunnis did the great input lag tests before, but I'm not sure if they still have an RPi to test with?

I guess this should negatively impact performance? I have been meaning to create a testing script that uses verbose console logs to track framerates on a batch of games - seems like a good excuse to write it :)

[vanfanel]

@dankcushions : Brunnis is busy at the moment. This will only impact performance if max_swapchain=2, with default max_swapchain=3 there's no impact at all. Supposing this works, which I can't prove without empirical evidence.