replace warning with comment in glz_usr_free_image
replace warning with comment in glz_usr_free_image

When running some xinerama tests, I got several
glz_usr_free_image: error
messages. Looking at the code, this error is reported when this
function is called from a different DisplayChannelClient than the
one which created the glz compressed image.
When this happens, the backtrace is
    at glz_encoder_dictionary.c:362
    0x7fff940b6670) at glz_encoder_dictionary.c:449
    image_type=LZ_IMAGE_TYPE_RGB32, image_width=512, image_height=256, image_stride=2048, first_lines=0x0,
    num_first_lines=0, usr_image_context=0x7fff7420da40, image_head_dist=0x7fff9b2a3194)
    at glz_encoder_dictionary.c:570
    top_down=4, lines=0x0, num_lines=0, stride=2048, io_ptr=0x7fff740ea7c0 "  ZL", num_io_bytes=65536, usr_context=
    0x7fff7420da40, o_enc_dict_context=0x7fff7420da60) at glz_encoder.c:255
    drawable=0x7fff9b46bc08, o_comp_data=0x7fff9b2a3350) at red_worker.c:5753
    0x7fff9b46bc08, can_lossy=0, o_comp_data=0x7fff9b2a3350) at red_worker.c:6211
    0x7fff9b46bc08, can_lossy=0) at red_worker.c:6344
    0x7fff74085c50, dpi=0x7fff7445b890, src_allowed_lossy=0) at red_worker.c:7046
    0x7fff7445b890) at red_worker.c:7720
    at red_worker.c:7964
    at red_worker.c:8431

Since the glz dictionary is shared between all the
DisplayChannelClient instances that belong to the same client, it can
happen that the glz dictionary code decides to free an image from one
thread while it was added from another thread (thread ==
DisplayChannelClient), so the error message that is printed is not an
actual error. This commit removes this message and adds a comment
explaining what's going on.

Fantu/spice,alon/spice-server,SPICE/spice,ya-mouse/spice,dezelin/spice,alon/spice-server,mathslinux/spice,SPICE/spice,alon/spice-server,ya-mouse/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,elmarco/spice,mathslinux/spice,freedesktop-unofficial-mirror/spice__spice,ya-mouse/spice,Fantu/spice,dezelin/spice,dezelin/spice,elmarco/spice,fgouget/spice,dezelin/spice,mathslinux/spice,freedesktop-unofficial-mirror/spice__spice,Fantu/spice,Fantu/spice,alon/spice-server,fgouget/spice,elmarco/spice,mathslinux/spice,SPICE/spice,freedesktop-unofficial-mirror/spice__spice,elmarco/spice,mathslinux/spice,fgouget/spice,elmarco/spice,alon/spice-server,dezelin/spice