Bug 1555511 Part 1: Make MVM::UpdateResolution clamp viewport zoom for the new display size later in the calculation. r=botond

The existing math is attempting to adjust the display scale ratio to
prevent the new size from landing outside the min/max zoom bounds. This
introduces unwanted side effects that can be avoided by simply clamping
to min/max at the end. The remaining math correctly handles the case
where the zoom has ALREADY been clamped, which is the only important
case.

Differential Revision: https://phabricator.services.mozilla.com/D32908

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extra : moz-landing-system : lando