Implement 4bpp graphics mode blink properly on VGA #3845
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Summary
This implements the 4bpp blink mode properly for real this time.
It turns out that if the plane 3 output is enabled in the graphics controller, then when it's 1, blink is enabled on that bit plane, and when it's 0, that bit plane's output is forced to... 1. Apparently someone thought this made sense.
Exactly what triggers it is something that will remain a mystery until I or someone willing to run tests actually gets access to a real VGA, but until then, this is the most logical explanation I can think of.
Checklist
References
Lotus 1-2-3 release 2.3 and QBASIC SCREEN 10. I'm not sure if any references explicitly document how graphics blink actually works, because from what I've read and remembered so far, IBM didn't. (The original implementation is based on reverse-engineering the mode tables in IBM's EGA documentation.)