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_gl.go

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// +build !windows
+
+package headless
+
+func newContext() (context, error) {
+	return newGLContext()
+}