enc-amf: Upgrade to v2.6

Deprecated support for drivers that ship an AMF Runtime older than
version 1.4.14, and older than 19.9.2. This also means Windows 8 is no
longer supported since AMD has stopped providing driver updates.

Added the new "High Motion Quality Boost" option that is exclusive to
the new 5700 cards and seems to do nothing on older cards.

Fixed the calculation of the automatic VBV buffer above 50%.

The Video API option will now only list one API, Direct3D 11 on Windows
10, and Direct3D 9 on Windows 7.

enc-amf @ 988437b5

-Subproject commit 47c3c728affbeb064779a7495818de73e727d2fb
+Subproject commit 988437b54b3b88d3ef3383a5b3d481ff51d32d51