internal/d3d11: extract Direct3D API to separate package

We're about the merge the Direct3D backend into package gpu. Extract the
raw Direct3D API to its own package, just like package glimpl.

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

app/internal/d3d11/backend.go

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// This file exists so this package builds on non-Windows platforms.
+
+package d3d11