Config: Correctly setup rgssVer / screen size with customScript
MRuby: Fix handling of Etc/Font properties

Port of f8c26fc515cb4fb6b24b766889d4b0b0a3c12a26.
MRI: WindowVX dummy contents object is always created

The check was against rgssVer >= 2, but since that's always
true if WindowVX is used, it was pointless.
Core, MRI: Don't expose RGSS3 functionality in 2