x265: updated to 2.7

Version 2.7

New features
1. :option:--gop-lookahead can be used to extend the gop boundary(set by --keyint). The GOP will be extended, if a scene-cut frame is found within this many number of frames.
2. Support for RADL pictures added in x265.
  :option:--radl can be used to decide number of RADL pictures preceding the IDR picture.

Encoder enhancements
1. Moved from YASM to NASM assembler. Supports NASM assembler version 2.13 and greater.
2. Enable analysis save and load in a single run. Introduces two new cli options --analysis-save <filename> and --analysis-load <filename>.
3. Comply to HDR10+ LLC specification.
4. Reduced x265 build time by more than 50% by re-factoring ipfilter.asm.

Bug fixes
1. Fixed inconsistent output issue in deblock filter and --const-vbv.
2. Fixed Mac OS build warnings.
3. Fixed inconsistency in pass-2 when weightp and cutree are enabled.
4. Fixed deadlock issue due to dropping of BREF frames, while forcing slice types through qp file.

(adam)