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/simple.vert

@@ -0,0 +1,20 @@
+#version 310 es
+
+// SPDX-License-Identifier: Unlicense OR MIT
+
+precision highp float;
+
+void main() {
+	float x, y;
+	if (gl_VertexIndex == 0) {
+		x = 0.0;
+		y = .5;
+	} else if (gl_VertexIndex == 1) {
+		x = .5;
+		y = -.5;
+	} else {
+		x = -.5;
+		y = -.5;
+	}
+	gl_Position = vec4(x, y, 0.5, 1.0);
+}