LibVideo/VP9: Implement a bit stream to decode VP9 data

The VP9 specification requires a special decoding process to parse a
lot of the data read from a frame, so this BitStream wrapper implements
that behavior. These processes are defined in section 9 of the
VP9 spec.

Userland/Libraries/LibVideo/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(SOURCES
     MatroskaDocument.h
     MatroskaReader.cpp
+    VP9/BitStream.cpp
     VP9/Enums.h
     VP9/ProbabilityTables.cpp
     VP9/Symbols.h