Added Angle-related enums and definitions

Added Angle-related enums and definitions

+ 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