Enable and test high bit depth input (#437)

* Attempt to process 10-bit Y4M input

Currently compiles, encodes and decodes with desynchronization

* Use high bit depth quantization tables

* Move context::clamp() to util.rs

* Fix partition context initialization for high bit depth

* Enable and test 10-bit input

* Add 10- and 12-bit test clips to build.sh

Commented out by default, to enable as needed for local testing.

* Use the same bit depth as y4m_dec for y4m_enc

* Fix benchmark module compilation

* Fix high bit depth test encoding in 8-bit

* Fix header syntax for 12-bit 4:2:0 input

* Enable and test 12-bit input

* Reflect 12-bit support in README.md