Add paint renderer compositor tests

src/paint_renderer/compositor.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include "foundation/result.h"
+#include "paint/blend.h"
+#include "renderer_api/renderer_api.h"
+
+#include <span>
+
+namespace pp::paint_renderer {
+
+struct LayerCompositeView {
+    std::span<const pp::paint::Rgba> pixels;
+    float opacity = 1.0F;
+    bool visible = true;
+    pp::paint::BlendMode blend_mode = pp::paint::BlendMode::normal;
+};
+
+[[nodiscard]] pp::foundation::Status composite_layer(
+    std::span<pp::paint::Rgba> destination,
+    pp::renderer::Extent2D extent,
+    LayerCompositeView layer) noexcept;
+
+}