WinRT: updated an OpenGL-related code-comment
WinRT: enabled OpenGL ES 2 support on Windows Phone

The "future-dev" branch of MSOpenTech's ANGLE/WinRT repository (at
https://github.com/msopentech/angle) includes support for Windows Phone 8.1.
This change allows it to be used in conjunction with SDL's OpenGL functions.
WinRT: fixed crash when trying, and failing, to load the opengles2 renderer

The crash would occur when a WinRT app explicitly tried to create an
SDL_Renderer using the "opengles2" renderer (via SDL_HINT_RENDER_DRIVER), but
OpenGL ES 2 / ANGLE .dlls weren't packaged in the app.