Add percentage to "Scale Type" to be able finetuning in smaller screens

[egorlins]

Thank you for making this app.
I have one device wit photo frame and this device is old phone.
As screen is too small I changed "Scale type" to Zoom, and some pictures are too cut.
if using "Fits" then a lot of black background for portrait pictures.

Cool if it will be adjustable, so I could fine tune it to my screen size.
the step of 10 or 20 percent is enough, I think.

"Fit" is 0 %
something between
"Zoom" is 100%

What do you think?