mark some init function as being safe for thread_sanitizer.

introduces the macro WEBP_TSAN_IGNORE_FUNCTION

Change-Id: I3de2b6c1a2076fba4da7ae50322551e026b2082b
add code for testing random incremental decoding in dwebp

(protected with WEBP_EXPERIMENTAL_FEATURES flag)

Change-Id: I6cc2488810ce2ccd5d45882bd12f34a3d0c8eab8