egl: add EGL_EXT_device_base entrypoints

egl: add EGL_EXT_device_base entrypoints

eglQueryDevicesEXT (unlike the other three functions) does not depend
on the display. It is implemented in GLVND, which calls into each
driver collecting the list of devices and presenting it to the user.

For the other entrypoints, GLVND acts as pass through stub calling into
the vendor library. The vendor implementation calls back into GLVND to
get the vendor dispatch. Then the driver proceeds to call itself via
the said dispatch.

This design makes is possible to keep using "old" GLVND with newer
vendor drivers. Since effectively all the extension code is within the
latter itself.

Without said entrypoints, any user will outright crash - as reported in
the bug report.

Note: there's a follow-up fix needed to our GLVND code, to make piglit
happy.

v2: add some beefy documentation in the commit message.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108635
Fixes: 7552fcb7b9b ("egl: add base EGL_EXT_device_base implementation")
Reported-by: kyle.devir@mykolab.com
Cc: kyle.devir@mykolab.com
Acked-by: Eric Engestrom <eric@engestrom.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Emil Velikov <emil.velikov@collabora.com>