Blank Screen Fix

This is a potential fix for a sneaky little bug which caused a completely blank screen, making the game completely unplayable.

With my current lack of a DOS hardware setup,  I've tested it in DOSBox-X and PCem, both of which had the bug before, so hopefully this should now work!

Thank you for your patience, and please let me know if you hit any bugs or other problems with it.

Technical details of bug, in case you're interested:

The bug was in my code for disabling the blinking of colours 8-15, which involved writing to VGA Attribute Register index 10h.

I was carefully following the process here, which is correct, but I had missed an important detail mentioned here (I actually noticed it in this pdf).

I was writing the index 10h which is required in bits 0-4, but in the process I was also inadvertently clearing bit 5 (Palette Address Source).  Bit 5 needs to be set to 1 for normal operation, hence the blank screen when I cleared it.  So the fix is basically writing 30h instead of 10h.  Lots of sweat for a tiny fix. :)