Be able to specify DPIs for a screenshot

[jhlegarreta]

Is your feature request related to a problem? Please describe.
Rather than a problem, it is a feature request. I think it would be desirable if the window.record or window.snapshot methods allowed to specify the DPI resolution of an image when it is to be saved. Not sure how this fits into the current framework where the size of the pictures is specified in pixels.

Describe the solution you'd like
Add an optional parameter to the window.record and window.snapshot methods allowing to specify the DPI resolution of te target file.

Describe alternatives you've considered
None applicable.

Additional context
Not sure if the underlying VTK classes allow to do this easily, or whether Pillow does allow it, and hence the option could be exposed.

[skoudoro]

Thank you for your feedback @jhlegarreta.

Following our calendar, this feature can be integrated for the release 0.9.0 (in March). However, feel free to give a try if you want it for an earlier release!