Weekly pre-release

Features:

• speed 0 is now up to 3x faster with no quality loss due to new early-exit implementation
• support for writing keyframe locations to a file for chunked encoders
• initial threading support (motion estimation only)
• tweaked quantizer modulation for better quality
• one-pass rate control
• 10% faster at speeds 3 and above due to addition of diamond me
• preparations for fast 8-bit mode

Fixes:

• bugfixes for HDR and colorspace obus (#977)