egl: implement EGL_EXT_image_flush_external
This is an extension from ChromiumOS, not present in the Khronos EGL registry. I don't know if the extension has to be added into the registry.
It's needed by radeonsi for transitioning compression from 3D to scanout. The important bits are:
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eglImageFlushExternalEXT
maps topipe_context::flush_resource
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EGL_IMAGE_EXTERNAL_FLUSH_EXT
maps toPIPE_HANDLE_USAGE_EXPLICIT_FLUSH