added refreshMath2img to the problem environment (PG.pm) and modified
Problem.pm to set it when showHints or showSolutions is on. THIS IS NOT
THE BEST THING TO DO -- we only need to refresh when the value of
showHints or showSolutions CHANGES. Also, we should be refreshing if the
problem seed changes. So, we need somewhere to store those three values
between invocations of the script. HHHHHMMMMMMMM......... &math2img in
dangerousMacros.pl has been modified to respect refreshMath2img and also
refresh if the image file is older than the PG source file.