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.
Potential fix for issue 3430

EnableCap.ColorArray is part of the client state and must be enabled
with GL.EnableClientState, not GL.Enalbe. This is a potential fix for
http://www.opentk.com/node/3430 "Picking example's problem"
Swallow DllNotFoundException in Sdl2 constructor
Merge branch 'master' of https://github.com/thefiddler/opentk into modes
Maintain support for VS2010 and MD2.x
Merge branch 'master' of https://github.com/thefiddler/opentk into modes
Use glXGetProcAddressARB

According to the Linux OpenGL ABI, glXGetProcAddressARB must be statically exported by libGL. This does *not* hold true for glXGetProcAddress. We must used the ARB version instead.
Furthermore, glx entry points, unlike wgl, do not depend on any specific OpenGL context. This means we can load them in the constructor of the Glx class.
Allow non-realized GraphicsMode

It is now possible to pass a non-realized GraphicsMode to the X11GLContext and X11GLNative constructors. A non-realized GraphicsMode is a GraphicsMode with a null Index (i.e. which has not passed through SelectGraphicsMode()).
Added mapping for libXi

On Debian-based distributions, libXi.so does not exist. We have to lookup libXi.so.6 instead.