Initial high DPI support, mostly for Android.

TODO: render fonts in their proper resolutions.
Where high DPI allows, render fonts in high resolution.

It looks beautiful.
Fix up some screen resize hiccups.

This is kind of hacky; for subscreens it works by passing a virtual
keystroke saying the screen needs to be resized.

It's not 100% bug-free yet, but it works for most purposes.
Mollify Travis with -lrt when building contrib sdl2.

We hope.
Use SDL_TEXTINPUT for most keys again.

Avoids awkward key mapping code. Hooray.