fixup picture in SetWindowPixmap

When creating a window with recordmydesktop running, the following may happen:

create picture 0x1cd457e0, with drawable 0x1327d1f0
(SetWindowPixmap is called)
destroy picture 0x1cd457e0, with drawable 0x1cd65820

Obtaining format for pixmap 0x1327d1f0 and picture 0x1cd457e0
==7989== Invalid read of size 4
==7989==    at 0x8CAA0CA: glamor_get_tex_format_type_from_pixmap (glamor_utils.h:1252)
==7989==    by 0x8CAD1B7: glamor_download_sub_pixmap_to_cpu (glamor_pixmap.c:1074)
==7989==    by 0x8CA8BB7: _glamor_get_image (glamor_getimage.c:66)
==7989==    by 0x8CA8D2F: glamor_get_image (glamor_getimage.c:92)
==7989==    by 0x29AEF2: miSpriteGetImage (misprite.c:413)
==7989==    by 0x1E7674: compGetImage (compinit.c:148)
==7989==    by 0x1F5E5B: ProcShmGetImage (shm.c:684)
==7989==    by 0x1F686F: ProcShmDispatch (shm.c:1121)
==7989==    by 0x15D00D: Dispatch (dispatch.c:432)
==7989==    by 0x14C569: main (main.c:298)
==7989==  Address 0x1cd457f0 is 16 bytes inside a block of size 120 free'd
==7989==    at 0x4C2B60C: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7989==    by 0x228897: FreePicture (picture.c:1477)
==7989==    by 0x228B23: PictureDestroyWindow (picture.c:73)
==7989==    by 0x234C19: damageDestroyWindow (damage.c:1646)
==7989==    by 0x1E92C0: compDestroyWindow (compwindow.c:590)
==7989==    by 0x20FF85: DbeDestroyWindow (dbe.c:1389)
==7989==    by 0x185D46: FreeWindowResources (window.c:907)
==7989==    by 0x1889A7: DeleteWindow (window.c:975)
==7989==    by 0x17EBF1: doFreeResource (resource.c:873)
==7989==    by 0x17FC1B: FreeClientResources (resource.c:1139)
==7989==    by 0x15C4DE: CloseDownClient (dispatch.c:3402)
==7989==    by 0x2AB843: CheckConnections (connection.c:1008)
==7989==
(II) fail to get matched format for dfdfdfdf

The fix is to update the picture pointer when the window pixmap is changed,
so it moves the picture around with the window rather than the pixmap.

This makes FreePicture work correctly.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71088
Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>
glamor: remove unused variable

Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>