Automatically enable and disable "Force Rasterization" when required

Okular implements various print scaling options that require the use of
the "Force rasterization" feature or else they have no effect. However
this is not communicated in the UI anywhere or handled automatically,
leading to users being confused and frustrated when the print scaling
option they chose didn't work. It wastes paper too.

This commit causes the "Force rasterization" checkbox to become checked
automatically whenever the user selects a print scaling option that
requires it, and unchecked if they return to the default scaling mode
that does not require it.

BUG: 434247
FIXED-IN: 21.08.2