Route RTT region readbacks through backend

src/rtt.cpp

@@ -503,6 +503,38 @@ glm::ivec4 RTT::calc_bounds() const noexcept
     return { bbmin, bbmax };
 }
 
+bool RTT::readPixelsRgba8(int x, int y, int width, int height, void* buffer) const noexcept
+{
+    if (!valid() || buffer == nullptr)
+        return false;
+
+    bool ret = true;
+    App::I->render_task([&]
+    {
+        const auto readback = pp::renderer::gl::rgba8_readback_format();
+        const auto status = pp::renderer::gl::readback_opengl_framebuffer(
+            pp::renderer::gl::OpenGlFramebufferReadback {
+                .framebuffer = static_cast<std::uint32_t>(fboID),
+                .x = x,
+                .y = y,
+                .width = width,
+                .height = height,
+                .format = readback,
+                .pixels = buffer,
+            },
+            pp::renderer::gl::OpenGlFramebufferReadbackDispatch {
+                .get_integer = query_opengl_integer,
+                .bind_framebuffer = bind_opengl_framebuffer,
+                .read_pixels = read_opengl_pixels,
+            });
+        if (!status.ok()) {
+            LOG("RTT::readPixelsRgba8() failed because: %s", status.message);
+            ret = false;
+        }
+    });
+    return ret;
+}
+
 uint8_t* RTT::readTextureData(uint8_t* buffer) const noexcept
 {
     if (!valid())