HM 源码分析（一）: Syntax Parse 函数解析

spec 中关于 ue 和 se 的计算有如下描述：

Depending on the descriptor, the value of a syntax element is derived as follows: If the syntax element is coded as ue(v), the value of the syntax element is equal to codeNum. Otherwise , the value of the syntax element is derived by invoking the mapping process for signed Exp-Golomb codes as specified in clause 9.2.2 with codeNum as input.

SyntaxElementParser相对应的是SyntaxElementWrite，其中包含了xWriteCode xWriteUvlc xWriteSvlc xWriteFlag四个函数。此处不在分析。