MEGA65: VIC-III (!) palette precision problem compared to the hardware

[lgblgblgb]

As it turned out in #256 , I have some palette precision problems when using "legacy" (VIC-III) palette registers, in terms of actual values represented in VIC-IV palette. Maybe other problems as well. Since it's kind of different topic than just reading back pixels in issue #256 , let's open a new issue on this specific topic.

New findings: palette handling in Xemu is somewhat wrong, by writing legacy VIC-III palreg with value $F (for example) results in $FF for Xemu for the real VIC-IV palreg (my idea was to allow to use the whole brightness spectrum via VIC-III as well). However it seems, on MEGA65 it's only done with writing the most significant nybble to set.

[lgblgblgb]

Now actual RGB values seems to be kinda identical with MEGA65 with the commit above. This also changed the palette slightly, which is not a problem, since it means it should be closer to MEGA65 now anyway ;)