layout: automatically add system inset for NewContext

Currently every user needs to manually adjust for system insets.
This is rather verbose and most don't need to deviate from this behavior.

To disable the automatic adjustment, use:

  e.Insets = system.Insets{}
  ctx := layout.NewContext(ops, e)

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

layout/context.go

@@ -5,6 +5,7 @@ package layout
 import (
 	"time"
 
+	"gioui.org/f32"
 	"gioui.org/io/event"
 	"gioui.org/io/system"
 	"gioui.org/op"
@@ -39,15 +40,30 @@ type Context struct {
 //     Constraints: Exact(e.Size),
 //   }
 //
-// NewContext calls ops.Reset.
+// NewContext calls ops.Reset and adjusts ops for e.Insets.
 func NewContext(ops *op.Ops, e system.FrameEvent) Context {
 	ops.Reset()
+
+	size := e.Size
+
+	if e.Insets != (system.Insets{}) {
+		left := e.Metric.Px(e.Insets.Left)
+		top := e.Metric.Px(e.Insets.Top)
+		op.Offset(f32.Point{
+			X: float32(left),
+			Y: float32(top),
+		}).Add(ops)
+
+		size.X -= left + e.Metric.Px(e.Insets.Right)
+		size.Y -= top + e.Metric.Px(e.Insets.Bottom)
+	}
+
 	return Context{
 		Ops:         ops,
 		Now:         e.Now,
 		Queue:       e.Queue,
 		Metric:      e.Metric,
-		Constraints: Exact(e.Size),
+		Constraints: Exact(size),
 	}
 }