Update to SDL2
Add missing mode flag
Add error handling for SDL_GL_CreateContext
Make sure fullscreen state is properly reflected in the UI
Add functionality to the MessagePopupScreen class.
It can more or less replace PromptScreen in PPSSPP.
Change OnChange back to OnChoice.
Not like it's used anyway, but it doesn't hurt.
base: Allow building when not using X11 on a FBDEV based system

* Currently building base will fail on non X11 based systems due to
  inclusion of X11 headers and some calls to X11 functions.
* Portions of the code currently attempts to avoid calling X11 functions
  when fbdev is used. This patch adds additional checks to avoid X11 calls.

Signed-off-by: Franklin S Cooper Jr <fcooper27@gmail.com>
Merge branch 'sdl2' of github.com:PGGB/native into PGGB-sdl2
Merge branch 'PGGB-sdl2'
Merge pull request #237 from thedax/messagepopupscreen

Add functionality to the MessagePopupScreen class.
Merge pull request #239 from fcooper/master-no-x11

base: Allow building when not using X11 on a FBDEV based system
Fix pause menu on Blackberry.
merged with upstream / amended SDL2 multi monitor support