egl: Add macros to define typecast functions.

egl: Add macros to define typecast functions.

There are standard typecast functions that are common to most drivers.
They are used to typecast, for example, an _EGLSurface to a
driver-defined type.

This commits define _EGL_DRIVER_STANDARD_TYPECASTS and
_EGL_DRIVER_TYPECAST that should hopefully save some typings for driver
writers.