Switching from windowed to full screen results in incorrect viewport on macs with retina displays about fna3d

[flibitijibibo]

This does seem like something that'd be specific to Cocoa, high-DPI in particular can be fussy, especially now that it's the default. If you set this at the top of main...

SDL2.SDL.SDL_SetHint("FNA_GRAPHICS_ENABLE_HIGHDPI", "1");

... does that work around it?

[briankendall]

@flibitijibibo No, that has no effect.

[flibitijibibo]

Understood - let's invalidate the cached viewport and scissor rectangles on resets within FNA3D then.

[flibitijibibo]

A minor detail worth noting for this fix is that zeroing the cached value will likely break things, as the value tends to be used in other areas like viewport flipping and deferred command buffer recording, so it's likely we'll just have to add a uint8_t forceVPUpdate that gets set to 1 on ResetBackbuffer.