Axis ticks are sometimes not aligned with canvas

[PierreRaybaut]

This issue was first discovered in guiqwt but it is a PythonQwt bug:
PlotPyStack/guiqwt#57

This issue can be reproduced with the ErrorBarDemo.py example. When running the example, the window first shows like this:

And then, when resizing the window even very slightly (here the width was reduced from a few pixels), everything is aligned: