Merge remote-tracking branch 'remotes/kraxel/tags/pull-sdl-20150505-1' into staging

sdl2: add opengl support

# gpg: Signature made Tue May  5 10:36:25 2015 BST using RSA key ID D3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <[email protected]>"
# gpg:                 aka "Gerd Hoffmann <[email protected]>"
# gpg:                 aka "Gerd Hoffmann (private) <[email protected]>"

* remotes/kraxel/tags/pull-sdl-20150505-1:
  sdl2: Fix RGB555
  sdl2: add support for display rendering using opengl.
  sdl2: move SDL_* includes to sdl2.h
  console-gl: add opengl rendering helper functions
  opengl: add shader helper functions.
  opengl: add shader build infrastructure

Signed-off-by: Peter Maydell <[email protected]>