wavefront: add a method to clear the enabled bitmask

--HG--
extra : amend_source : cfe074ec3a49b6a7bfb4588747a4c5db0c059ec4
frameencoder: clear enabled row bitmask at the start of each frame
common: prune default search length to 60, for optimal frame parallelism
TypeDef: give explicit namespace to X265_DEPTH for 16bpp builds
framepp: number of frame threads always less than lookahead depth