dri3: Fix dri3_open API change by adding new dri3_open_client

Xwayland will eventually need the current client in dri3_open. Simply
changing that API is not an option though as other drivers that
implement DRI3 will not have a matching function signature and will
crash when called.

Add a new dri3_open_client function pointer and bump
DRI3_SCREEN_INFO_VERSION so that drivers can be aware of the new
function which will be used in preference to the old function when
available.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Eric Anhole <eric@anholt.net>
Bump to version 1.15.99.902. Closing the 1.16 merge window.

Signed-off-by: Keith Packard <keithp@keithp.com>