feat: add retained underlay render cache poc

shaders/retained_underlay_composite.vertex.glsl

@@ -0,0 +1,7 @@
+layout (location = 0) in vec2 a_pos;
+out vec2 v_uv;
+
+void main() {
+    gl_Position = vec4(a_pos.x * 2.0 - 1.0, 1.0 - a_pos.y * 2.0, 0.0, 1.0);
+    v_uv = vec2(a_pos.x, 1.0 - a_pos.y);
+}