Magnifier Improvements

[dkaraush]

Few improvements on Accessibility > Magnifier feature:

• Added "Smoothing" toggle in settings: when disabled, shows true pixels of zoomed display image (useful for designing pixel perfect)
• When magnifying in fullscreen, first zooms allowed to zoom out of bounds (later cursor movements don't), now this is fixed.
• Cursor preview and position was not scaled correctly with fractional scale, apply also disabled smoothing to cursor sprite
• Fixed magnifier breaking on display resolution/scale change

Fixes are tested against current upstream of cinnamon&muffin before this PR.

Also related to magnifier, but fix for it is in muffin: linuxmint/muffin#838