[EGL] Egl.GetProcAddress only returns extensions

We have to load statically exported entry points using the regular OS
symbol loading facilities, and only use eglGetProcAddress for
extensions.

Fixes crash in https://github.com/mono/MonoGame/pull/2377
[EGL] Fixed display connection

We now use the display connection specified in EglWindowInfo
throughout the EGL backend, instead of opening new connections
in EglGraphicsMode. This fixes EGL errors in
Egl.CreateWindowSurface.

As part of this fix, all instances of
IPlatformFactory.CreateGraphicsMode() have been removed. This
API is no longer used in OpenTK 1.1
Removed file committed by mistake
Merge pull request #123 from thefiddler/eglfix

[EGL] Fix initialization and extension loading