Is there graphical editor for mac supporting non linear distortions (stretch)?

I am a software developer facing a rare need to create single PNG icon. It should be a horizontal gradient (white to transparent) rectangle stretched to the right - so that right edge of the rectange will become quadratic bezier curve. The gradient should also be stetched.

I thought this is very simple to do but I just spend a day on it without any progress. I haven't found graphical editor that would support non-linear (non-affine) transformation of shapes.

I would be very gratefull for any help.