mdec.cpp: Dare to set MDEC_BIAS to 10

This fixes graphic artifacts during cinematics in :
- Vandal Hearts
- R-Types
- Galerians

Dmitrysmagin had a similar fix in PCSX4ALL but he set it to 6.
It turns out MDEC_BIAS set to 6 was still not enough for Galerians
to look properly so it was set to 10.
It seems so far that other games are unaffected.

Co-authored-by: Dmitry Smagin <exmortis@yandex.ru>