no exec? - we try proc_open() and other alternatives

• When exec() is unavailable, alternatives are now tried (emulations with proc_open(), passthru() etc). Using this library to do it.
• Gd is now marked as not operational when the needed functions for converting palette images to RGB is missing. Rationale: A half-working converter causes more trouble than one that is marked as not operational
• Improved CI tests. It is now tested on Windows, Mac and with deactivated functions (such as when exec() is disabled)
• And more (view closed issues here