Put Luma instead of CVBS on the pin-4 of the RGB connector

[sdsnatcher]

The current revision of the OmegaMSX has CVBS (aka "Composite Video") on the pin-4 of the RGB connector.

But this is known to cause jailbars on the RGB image.

It's better to replace it with the Luma signal from the S-Video connector (pin-3). This is a much better way to provide a cheap/clean CSYNC signal for the RGB connector.

[skiselev]

Sounds like a reasonable fix. Adding to the next version improvements