widget: fix image scaling

Commit 94d242d broke the widget.Image's Scale field so
that it no longer had any effect on the actual size of
the displayed image. This commit fixes that, as well as
adding tests to confirm that the widget.Image type
scales appropriately with DPI changes and its own Scale
field.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/image.go

@@ -5,6 +5,7 @@ package widget
 import (
 	"image"
 
+	"gioui.org/f32"
 	"gioui.org/layout"
 	"gioui.org/op"
 	"gioui.org/op/clip"
@@ -22,10 +23,12 @@ type Image struct {
 	Scale float32
 }
 
+const defaultScale = float32(160.0 / 72.0)
+
 func (im Image) Layout(gtx layout.Context) layout.Dimensions {
 	scale := im.Scale
 	if scale == 0 {
-		scale = 160.0 / 72.0
+		scale = defaultScale
 	}
 	size := im.Src.Size()
 	wf, hf := float32(size.X), float32(size.Y)
@@ -33,6 +36,8 @@ func (im Image) Layout(gtx layout.Context) layout.Dimensions {
 	cs := gtx.Constraints
 	d := cs.Constrain(image.Pt(w, h))
 	stack := op.Save(gtx.Ops)
+	pixelScale := scale * gtx.Metric.PxPerDp
+	op.Affine(f32.Affine2D{}.Scale(f32.Point{}, f32.Pt(pixelScale, pixelScale))).Add(gtx.Ops)
 	clip.Rect(image.Rectangle{Max: d}).Add(gtx.Ops)
 	im.Src.Add(gtx.Ops)
 	paint.PaintOp{}.Add(gtx.Ops)