Replace explicit DPI conversions with standard unit type

[oshorefueled]

running $ grep "\.PxPer

returns a few instances of explicit use of the pixel-to-dp scale. The pageCommon.layoutNavDrawer converts from Dps to pixels:

gtx.Constraints.Min.X = int(gtx.Metric.PxPerDp) * width

However, PxPerDp may not be integral; a better version is

gtx.Constraints.Min.X = int(gtx.Metric.PxPerDp * width)

Even better is to change the type of width, navDrawerWidth, navDrawerMinimizedWidth to unit.Value, and use

gtx.Constraints.Min.X = gtx.Px(width)

A related issue is the godcr/ui/decredmaterial.pxf function; it is a copy of gioui.org/unit.Metric.Px except that it special-cases the zero scale. I think it is better to never construct zero-valued unit.Metrics and use the standard Px method instead.

This is in reference to Elias' review: https://paste.sr.ht/~eliasnaur/cea1d29d6a5f96668b5e166c2f39ef596974574f