WinRT: expanded OpenGL ES 2 support to enable recent updates to ANGLE/WinRT

ANGLE for WinRT has at least two versions:
- an older version, which supports Windows 8.0 and 8.1.  This is currently
  the "winrt" branch in MSOpenTech's ANGLE repository (at
  https://github.com/msopentech/angle)
- a newer version, which drops support for Windows 8.0, but is under more
  active development (via MSOpenTech's "future-dev" branch), and which was
  recently merged into the ANGLE project's official "master" branch
  (at https://chromium.googlesource.com/angle/angle)

Both versions are setup using slightly different APIs.  SDL/WinRT will now
attempt to detect which version is being used, and configure it appropriately.

--HG--
extra : rebase_source : aba4c5ec99d0e13cff0c7a4c842c4d9d322e45e3