contrib: update libvpx to 1.11.0

2021-09-27 v1.11.0 "Smew Duck" This maintenance release adds support for VBR mode in VP9 rate control interface, new codec controls to get quantization parameters and loop filter levels, and includes several improvements to NEON and numerous bug fixes.

Upgrading:
New codec control is added to get quantization parameters and loop filter
levels.

VBR mode is supported in VP9 rate control library.

Enhancement:
Numerous improvements for Neon optimizations.
Code clean-up and refactoring.
Calculation of rd multiplier is changed with BDRATE gains.

Bug fixes:
Fix to overflow on duration.
Fix to several instances of -Wunused-but-set-variable.
Fix to avoid chroma resampling for 420mpeg2 input.
Fix to overflow in calc_iframe_target_size.
Fix to disallow skipping transform and quantization.
Fix some -Wsign-compare warnings in simple_encode.
Fix input file path in simple_encode_test.
Fix valid range for under/over_shoot pct.

Source: https://github.com/webmproject/libvpx/releases/tag/v1.11.0