The graphicsPort property of NSGraphicsContext is deprecated in 10.14.
CGContext is there since 10.10 so we can use it unconditionally.

But sadly this did not enable me to un-comment the assert() a few
lines below.

Wonder if this deprecation is just a cleanup of the API, and
functionality-wise the CGContext property is exactly the same as
graphicsPort?

Change-Id: I2d5d0c766c343faa2d8cabea3a0729ce5b3a34e3