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>

gpu/headless/headless_windows.go

@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package headless
+
+import (
+	"unsafe"
+
+	"gioui.org/gpu"
+	"gioui.org/internal/d3d11"
+)
+
+type d3d11Context struct {
+	dev *d3d11.Device
+}
+
+func newContext() (context, error) {
+	dev, ctx, _, err := d3d11.CreateDevice(
+		d3d11.DRIVER_TYPE_HARDWARE,
+		0,
+	)
+	if err != nil {
+		return nil, err
+	}
+	// Don't need it.
+	d3d11.IUnknownRelease(unsafe.Pointer(ctx), ctx.Vtbl.Release)
+	return &d3d11Context{dev: dev}, nil
+}
+
+func (c *d3d11Context) API() gpu.API {
+	return gpu.Direct3D11{Device: unsafe.Pointer(c.dev)}
+}
+
+func (c *d3d11Context) MakeCurrent() error {
+	return nil
+}
+
+func (c *d3d11Context) ReleaseCurrent() {
+}
+
+func (c *d3d11Context) Release() {
+	d3d11.IUnknownRelease(unsafe.Pointer(c.dev), c.dev.Vtbl.Release)
+	c.dev = nil
+}