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/shaders/input.vert

@@ -0,0 +1,11 @@
+#version 310 es
+
+// SPDX-License-Identifier: Unlicense OR MIT
+
+precision highp float;
+
+layout(location=0) in vec4 position;
+
+void main() {
+	gl_Position = position;
+}