GDK/Win32: Try to fix initializing GLES contexts

GDK/Win32: Try to fix initializing GLES contexts

We are now able to create EGL contexts properly on Windows, but not GLES.  This
tries to fix things by doing the following:

*  Record the GL context type in a more proper fashion, using an Enum.  This
   makes things a bit cleaner.
*  Force GLES-3.0+ contexts, since libANGLE requires this to properly work with
   the shaders-its 2.0 contexts don't work well with our shaders.