llvmpipe: support pipe_resource-based constant buffers

llvmpipe: support pipe_resource-based constant buffers

Before this we only supported user-based constant buffers.

First, we basically plumb pipe_constant_buffer objects through llvmpipe
rather than pipe_resource objects.

Second, update llvmpipe_set_constant_buffer() and try_update_scene_state()
so they understand both resource- and user-based constant buffers.

The problem with user constant buffers is the potential for use-after-free,
as seen in some WebGL tests.  The next patch will flip the switch for
resource-based const buffers.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>