app/headless: add lower-level backend tests

Add a series of low level gpu.Backend tests to assure the correct behaviour of
Backends. The immediate use is debugging of the Direct3D port, in the future
for developing new backends.

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

app/headless/gen.go

@@ -0,0 +1,5 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package headless
+
+//go:generate go run ../../internal/cmd/convertshaders -package headless