Fix python to C++ pointer conversion issue in moldrawFromQPainter

[rachelnwalker]

On 64-bit windows, I noticed that the value of ptr/pointer_to_QPainter can exceed the maximum unsigned long int value. In some cases, this caused an OverflowError when using MolDraw2DFromQPainter to make a PNG. I think that changing the ptr type to size_t should prevent this issue on all platforms, but there might be a better approach.

Here is the traceback I was getting:

    drawer = MolDraw2DFromQPainter(cpp_qp, options.width, options.height)
...\rdkit\Chem\Draw\__init__.py:177: in MolDraw2DFromQPainter
    d2d = rdMolDraw2DQt.MolDraw2DFromQPainter_(width, height, ptr, panelWidth, panelWidth)
E   OverflowError: Python int too large to convert to C unsigned long
--------------------- Captured stdout call -----------------------------
QPainter address: <PyQt5.QtGui.QPainter object at 0x000002A0856282E0>

[greglandrum]

Thanks @rachelnwalker!