Add support for Hi-DPI displays

[julian37]

It would be nice if 0 A.D. could take advantage of Hi-DPI displays, such as Apple Retina's displays.

As far as I can tell, this requires the following:

• Upgrading to SDL2, which adds the SDL_WINDOW_ALLOW_HIGHDPI flag
• Ensuring the GUI remains at the original size (i.e. magnifying it by 2x with respect to 3D graphics)

And the following are nice-to-haves:

• Doubling the resolution of the prerendered fonts for crisp text rendering
• Providing all bitmap graphics in double the resolution (seeing that 4K displays are on the horizon, perhaps new bitmaps graphics should be prepared at 4x the resolution)

I've attached a simple-minded patch for the first issue (SDL2 support) to ticket #2041, and I've prepared a patch to address issues 2 and 3 (scaled-up GUI and high-res fonts), will attach it shortly.