Allow maxGUI to be used without taking over the entry point

[ProgramMax]

Currently, maxGUI requires controlling the entry point. This is handy to standardize different platform entry points, resulting in user code being more platform independent.

However, many people would prefer to maintain control of the entry point themselves. Or if another API/framework itself requires control, it becomes incompatible with maxGUI.

Really, maxGUI could have an init function. The entry point standardization can just be a convenience option.