Update to 2.6.0

Changelog:
2.6.0
Align libva attribute check
Added displaying supported config attributes
add the VP samples README.
add VP sample for usrptr and 1:N output

2.5.0
meson: rename the h264enc binary to be in line with the autotools build
unify NEWS format
sfcsample: don't include X11 headers
test: update vaInitialize_vaTerminate_Bad_vaSetDriverName
Check for -fstack-protector
vavpp: csc with AYUV format
vavpp: requires BGRA frame when saving a BGRA file

2.4.0
fix make dist issue caused by decode streamout sample
add VP sample for Blending.
add VP seperate sample for scaling,csc,sharpness,denoise,chromasitting.

(ryoon)