machine/z80scc.cpp: Fixed a cast-to-bool that broke detection of changes
to one register bit.
formats/fsmeta.cpp: Use visitors with variants where it makes sense.
docs: Updated minimum required SDL version to 2.0.6 for all targets,
added note that Python 3 is included with Xcode and updated instructions
for downloading stand-alone Python 3 for macOS.
ksys573.cpp: Don't create an insane number of textures for fghtmn and
pnchman internal artwork.
Tidied another batch of slot machine layouts.
6ff40e09bd