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
PlatformFactoryBase provides a common base interface for platform
backends. Platform backends should inherit from PlatformFactoryBase in
order to reduce code duplication.