cogland: Use wl_resource_get_user_data instead of resource->data
cogland: Use wl_resource_get_user_data instead of resource->data

wl_resource may become an opaque type in future so we should probably
be using the accessors instead of directly prodding into the struct.

Reviewed-by: Robert Bragg <12e9293ec6b30c7fa8a0926af42807e929c1684f@linux.intel.com>

djdeath/cogl,gcampax/cogl,gcampax/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl,djdeath/cogl