It's a music notation library for displaying musical scores (http://www.music-software-development.com/music-notation-software-library.html) and this is what is stated about displaying the score:
This music notation software library function displays the given page of the score in page mode, from the specified open document.
For Windows, drawing is performed to the device context supplied by the graphic parameter. The graphic parameter is in fact a Windows HDC type (handle to a device context). The most obvious way to use this function is while handling the WM_PAINT Windows message.
For Mac, the graphic parameter is in fact an NSView object or a sub-class of it. The most obvious place to use this function is in the implementation function of the NSView object.
From this I understood that that "graphic" input parameter is a HDC on Windows.
So I guess that since I can get the window handle with getWindowHandle as you say, I can simply call GetDC(HWND hWnd) on Windows to get the DC and hand that over. Unless that's going to cause all kinds of problems for some reasons?