gpu/headless: move package app/headless

Package headless is more about rendering that windows. Move it
accordingly.

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

internal/rendertest/bench_test.go

@@ -6,9 +6,9 @@ import (
 	"math"
 	"testing"
 
-	"gioui.org/app/headless"
 	"gioui.org/f32"
 	"gioui.org/font/gofont"
+	"gioui.org/gpu/headless"
 	"gioui.org/layout"
 	"gioui.org/op"
 	"gioui.org/op/clip"

internal/rendertest/util_test.go

@@ -17,8 +17,8 @@ import (
 
 	"golang.org/x/image/colornames"
 
-	"gioui.org/app/headless"
 	"gioui.org/f32"
+	"gioui.org/gpu/headless"
 	"gioui.org/internal/f32color"
 	"gioui.org/op"
 	"gioui.org/op/paint"