It's possible for new devices to become available while a program is running,
so eglQueryDevicesEXT might need to return a different list.

Change the EGLDeviceEXT hashtable so that each entry is allocated and added
separately, rathat than allocating every entry in a single malloc.

Added a new __eglAddDevice function, which adds an entry to the device
hashtable.

When the application calls eglQueryDevicesEXT, always call through to each
vendor library, and add (or update) each EGLDeviceEXT to the hashtable.

There's still no provision for removing an entry from the device hashtable. The
semantics of hot-unplugging a device aren't defined yet, and there's no way to
ensure that libglvnd's mapping will get updated if a device disappears
asynchonously, so it's up to the vendor library to deal with the old handle.