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_egl.go

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// +build linux freebsd windows openbsd
+
+package headless
+
+import (
+	"gioui.org/internal/egl"
+)
+
+func newGLContext() (context, error) {
+	return egl.NewContext(egl.EGL_DEFAULT_DISPLAY)
+}