Implement `m_FlagsEx` from CS:GO's static props

[PiMoNFeeD]

It is used for disabling shadow depth and enabling light bounces. You mentioned in README that it is lost on compilation but in fact it isn't; I just made a test map with some props having enabled static prop bounced lighting and some not, and having manually implemented m_FlagsEx the output is correct. In fact the entire m_FlagsEx field is saved in the BSP file, although there is absolutely no condition in tools to toggle STATIC_PROP_FLAGS_EX_DISABLE_CSM, so you can skip that one.