VP8: allow thread count changes

VP8: allow thread count changes

Fix the TODO(https://crbug.com/1486441) comment in vp8/vp8_cx_iface.c.

Make vp8cx_create_encoder_threads() work after it has been called
before. If there are already the exact number of threads it needs to
create, return immediately. Otherwise, shut down the existing threads
(by calling vp8cx_remove_encoder_threads()) and create the required
number of threads.

Call vp8cx_create_encoder_threads() in vp8e_set_config() to respond to
changes in g_threads or g_w (which also affects the number of threads
through cm->mb_cols and cpi->mt_sync_range).

Change-Id: I552eeca5b1f1f5313f59559eb1da396f270a2429