Fix artefact in Qt caused by QRectF having a slightly wrong size

[kitchoi]

Closes #819

The issue is tested (manually) to affect these backends: qt4.image, qt4.celiagg and qt4.cairo.
For these backends, the GraphicsContext is defined to be slightly larger than the requested size:

enable/enable/qt4/cairo.py

Lines 20 to 23 in 1c3805c

	gc = GraphicsContext(
	(size[0] + 1, size[1] + 1),
	base_pixel_scale=self.base_pixel_scale,
	)

There is another QRectF defined for qt4.quartz, but I could not reproduce the issue there. For this backend, the GraphicsContext was defined without the plus-one:

enable/enable/qt4/quartz.py

Line 35 in 6850385

gc = GraphicsContext(size)

Note that base_scale was not defined on the GraphicsContext for the cairo backend, and so it has to be added.

Apparently base_scale is not defined on the qpainter's GraphicsContext either, but that would be an orthogonal issue as #819 does not affect the qpainter backend.

[rahulporuri]

@jwiggins any chance you can make time to review this PR today/monday? It looked good to me but i wanted a more experienced pair of eyes. The idea is to make a bug fix release ASAP once this is merged.

[jwiggins]

LGTM

[rahulporuri]

@kitchoi i might be doing something wrong but have you/someone else on your team tested this fix on windows? I was in fact doing something wrong. Tested this locally on windows with the example code in #819 . Tested against image and celiagg backends. Works as expected.

[rahulporuri]

LGTM

[kitchoi]

Thanks for the review and testing!