gallium/util: add threaded_context as a pipe_context wrapper

gallium/util: add threaded_context as a pipe_context wrapper

v2: - rename num_calls -> num_call_slots (for tc_call)
    - rename num_calls -> num_total_call_slots (for tc_batch)
    - rename num_offloaded/direct_calls -> num_offloaded/direct_slots
    - declare slot[0] instead of slot[1]
    - remove no-op leftover code from tc_draw_vbo
    - use tc_set_resource_reference to fill threaded_transfer
    - fix map flags for sparse buffers
    - cosmetic changes

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>