Add more checks for checkpoint saving

[NyakoFox]

Changes:

Checkpoint saving no longer happens if:

• You're in custommode, but not custommodeforreal (editor playtesting)
• You're in cliplaytest (playtest mode, but it DOES work if you just pass the play flag without a save)

Fixes #1198.

Legal Stuff:

By submitting this pull request, I confirm that...

• My changes may be used in a future commercial release of VVVVVV
• I will be credited in a CONTRIBUTORS file and the "GitHub Friends"
  section of the credits for all of said releases, but will NOT be compensated
  for these changes unless there is a prior written agreement