assert() abort with ETC2A1 & RGBX

There is an internal inconsistency in the handling of `m_boolPunchThroughAlpha` [here](https://github.com/google/etc2comp/blob/master/EtcLib/EtcCodec/EtcBlock4x4Encoding_RGB8A1.cpp#L1664) that triggers with the RGBX error metric.

We could probably fix it fairly easily, but there's little indication that pull requests are being accepted here anymore. Is there a fork that's actively maintained?
