Update drawing.cpp

Update drawing.cpp

Update EHB logic so palette mirroring will be correct with A1000 early Denise chip.  With AGA chipset, bplehb may be zero (uses color registers 32-63), but for OCS/ECS it must be -1 (mirrors color registers 0-31).  Logic assumes that OCS Denise with EHB support cannot disable its EHB colors.