This fix will keep existing behavior (forced sharing) in the default case, but adds a new constructor that allows explicitly specifying the shared context. A user can now explicitly specify null for the shared context to get a new non-shared context.
Both WindowInfo and PlatformFactory wrap the actual platform-specific WindowInfo and PlatformFactory.
For offscreen rendering, AngleWindowInfo can be used with a DummyWindowInfo.
The API to add additional surfaces was added to IAngleWindowInfo, since all those calls need the Display parameter that is only available on EglWindowInfo (which is not exposed to users).
+ GraphicsContextFlags now has Angle and Offscreen flags.
+ Egl now has PLATFORM_ANGLE related const ints.
+ Added eglGetPlatformDisplayEXT p/invoke definition
+ Added eglQuerySurfacePointerANGLE p/invoke definition
+ EglWindowInfo.CreatePbufferSurface was commented out. Enabled and fixed it.
+ GraphicsContextFlags.Offscreen will make the context create a PbufferSurface instead of a WindowSurface in EglContext
+ SurfaceType enum to select surface type for SelectGraphicsMode
