all: make unit.Converter concrete and rename to Metric

An interface for scaling dp and sp is overkill, at least for all
current uses. Make it a concrete struct type, and rename it to the
shorter and more precise Metric.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/window/window.go

@@ -6,7 +6,6 @@ package window
 
 import (
 	"errors"
-	"math"
 
 	"gioui.org/gpu/backend"
 	"gioui.org/io/event"
@@ -71,29 +70,6 @@ type windowAndOptions struct {
 	opts   *Options
 }
 
-// config implements the system.Config interface.
-type config struct {
-	// Device pixels per dp.
-	pxPerDp float32
-	// Device pixels per sp.
-	pxPerSp float32
-}
-
-func (c *config) Px(v unit.Value) int {
-	var r float32
-	switch v.U {
-	case unit.UnitPx:
-		r = v.V
-	case unit.UnitDp:
-		r = c.pxPerDp * v.V
-	case unit.UnitSp:
-		r = c.pxPerSp * v.V
-	default:
-		panic("unknown unit")
-	}
-	return int(math.Round(float64(r)))
-}
-
 func newWindowRendezvous() *windowRendezvous {
 	wr := &windowRendezvous{
 		in:   make(chan windowAndOptions),