Fixes ArcticaProject/nx-libs#598

In nxagentOpenScreen we first initialized the RRExtension for the
screen and then replaced pScreen->CloseScreen by
nxagentCloseScreen. This resulted in RandR's RRCloseScreen (and any
other CloseScreen procedure installed by extensions) being no longer
called.

Moving RandR init after configuring pScreen->CloseScreen ensures the
correct calling cascade:

RRCloseScreen -> nxagentCloseScreen ->fbCloseScreen (called explicitly
by nxagentCloseScreen).

Which in turn will fix this memory leak:

==9688== 328 (312 direct, 16 indirect) bytes in 1 blocks are definitely lost in loss record 271 of 319
==9688==    at 0x4837B65: calloc (vg_replace_malloc.c:752)
==9688==    by 0x4ED2C6: RRScreenInit (randr.c:329)
==9688==    by 0x1F2B18: nxagentInitRandRExtension (Extensions.c:122)
==9688==    by 0x1DEAFF: nxagentOpenScreen (Screen.c:1409)
==9688==    by 0x16D7F8: AddScreen (dispatch.c:4257)
==9688==    by 0x1DA0CF: InitOutput (Init.c:397)
==9688==    by 0x14DCC2: main (main.c:280)