examples: add render-pass-ext exmaple

examples/render-pass-ext/vulkan/shaders/triangle.vert

@@ -0,0 +1,15 @@
+#version 450
+
+// Keep push constants layout compatible with wlroots Vulkan helpers.
+layout(push_constant, row_major) uniform UBO {
+	mat4 proj;
+	vec4 pos[3];
+	vec4 color[3];
+} data;
+
+layout(location = 0) out vec3 v_color;
+
+void main() {
+	gl_Position = data.proj * vec4(data.pos[gl_VertexIndex].xy, 0.0, 1.0);
+	v_color = data.color[gl_VertexIndex].rgb;
+}