EGL: resync platforms with Khronos registry and rationalize build rules
window: add window type

EGL needs to know the kind of window at run-time.
EGL: build separate plugins for each windowing system

This is required to support non-native EGL platforms.
EGL: check window type and revector
EGL: support non-default X11 display
EGL: add CheckClientExt() helper

Checks support for a EGL client extension.
EGL: add support for EGL_EXT_platform_base and EGL_EXT_platform_x11