ui/app: introduce Insets for system decoration insets

Insets is like image.Rectangle, but with properly named fields
and ui.Value instead of raw ints to make use with the layout
package easier.

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

ui/app/app.go

@@ -19,16 +19,20 @@ type Event interface {
 type DrawEvent struct {
 	Config Config
 	Size   image.Point
-	// Insets is the window space taken up by
-	// system decoration such as translucent
-	// system bars and software keyboards.
-	Insets image.Rectangle
+	Insets Insets
 	// Whether this draw is system generated
 	// and needs a complete frame before
 	// proceeding.
 	sync bool
 }
 
+// Insets is the space taken up by
+// system decoration such as translucent
+// system bars and software keyboards.
+type Insets struct {
+	Top, Bottom, Left, Right ui.Value
+}
+
 type StageEvent struct {
 	Stage Stage
 }

ui/app/os_android.go

@@ -24,6 +24,7 @@ import (
 	"time"
 	"unsafe"
 
+	"gioui.org/ui"
 	"gioui.org/ui/f32"
 	"gioui.org/ui/key"
 	"gioui.org/ui/pointer"
@@ -36,7 +37,7 @@ type window struct {
 
 	dpi       int
 	fontScale float32
-	insets    image.Rectangle
+	insets    Insets
 
 	stage   Stage
 	started bool
@@ -200,9 +201,11 @@ func onFocusChange(env *C.JNIEnv, class C.jclass, view C.jlong, focus C.jboolean
 //export onWindowInsets
 func onWindowInsets(env *C.JNIEnv, class C.jclass, view C.jlong, top, right, bottom, left C.jint) {
 	w := views[view]
-	w.insets = image.Rectangle{
-		Min: image.Point{X: int(left), Y: int(top)},
-		Max: image.Point{X: int(right), Y: int(bottom)},
+	w.insets = Insets{
+		Top: ui.Px(float32(top)),
+		Right: ui.Px(float32(right)),
+		Bottom: ui.Px(float32(bottom)),
+		Left: ui.Px(float32(left)),
 	}
 	if w.stage >= StageRunning {
 		w.draw(true)

ui/app/os_ios.go

@@ -22,6 +22,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"gioui.org/ui"
 	"gioui.org/ui/f32"
 	"gioui.org/ui/key"
 	"gioui.org/ui/pointer"
@@ -82,9 +83,11 @@ func onDraw(view C.CFTypeRef, dpi, sdpi, width, height C.CGFloat, sync C.int, to
 			X: int(width + .5),
 			Y: int(height + .5),
 		},
-		Insets: image.Rectangle{
-			Min: image.Point{X: int(left + .5), Y: int(top + .5)},
-			Max: image.Point{X: int(right + .5), Y: int(bottom + .5)},
+		Insets: Insets{
+			Top:    ui.Px(float32(top)),
+			Right:  ui.Px(float32(right)),
+			Bottom: ui.Px(float32(bottom)),
+			Left:   ui.Px(float32(left)),
 		},
 		Config: Config{
 			pxPerDp: float32(dpi) * inchPrDp,