Load ES30 and OpenGL4 entry points
Remove static init of GetCurrentContext

The static initializer of GetCurrentContext would always default to the
desktop PlatformFactory, which caused problems when trying to run OpenGL
ES code on the desktop. The initializer is now removed and
GetCurrentContext is set in the context constructor, before creating any
contexts.
Stronger check for ES support

It is not enough to check for EGL, we also need to check that
ContextProfileFlags.ES is set.
Updated WGL init sequence

WinGraphicsMode no longer creates a temporary context in order to create
the list of available modes. Instead, it requires to be passed an
existing context in its constructor.

WinGLContext now creates one temporary context in its static constructor
and hands that to WinGraphicsMode.

WinFactory no longer supports the CreateGraphicsMode API. This API will
be removed in the future, because the link because contexts and modes
cannot be separated in the general case.