Merge multiple egl_<platform>_<pipe>.so into a single
egl_gallium_<pipe>.so. The environment variable EGL_PLATFORM is now
used to modify the return value of _eglGetNativePlatform.
This commit introduces type-safe platform displays internally. A
platform display consists of a generic pointer and an enum that
specifies the platform.
An EGLDisplay is created from a platform display. Native displays
become platform displays whose platform is determined by
_eglGetNativePlatform(). Platform windows and pixmaps may also be
introduced if needed.