Use gswin64c instead of gswin64 to avoid popup windows at each draw() command

In windows 64 bits gswin64.exe is used for ghostscript.
This causes a window to popup and close after each draw command.
Solution: Use gswin64c.exe instead.

I suggest to modify the line 131 system.py:

Before:
program = 'gswin64'

after:
program = 'gswin64c'

Alternatives:
Change the code to allow users to change the program name in their code (like manipulating the dict 'programs' at system.py).

