Generate bindings of Image::to_rgb8/to_rgba8/to_rgba8_premultiplied f… (#6405)

ChangeLog: [C++] Add `Image::to_rgb8/to_rgba8/to_rgba8_premultiplied` pixel buffer accessors.

Fixes #6399

api/cpp/tests/datastructures.cpp

@@ -200,6 +200,31 @@ TEST_CASE("Image")
     }
 }
 
+TEST_CASE("Image buffer access")
+{
+    using namespace slint;
+
+    auto img = Image::load_from_path(SOURCE_DIR "/redpixel.png");
+
+    REQUIRE(!img.to_rgb8().has_value());
+
+    {
+        auto rgb = img.to_rgba8();
+        REQUIRE(rgb.has_value());
+        REQUIRE(rgb->width() == 1);
+        REQUIRE(rgb->height() == 1);
+        REQUIRE(*rgb->begin() == Rgba8Pixel { 255, 0, 0, 255 });
+    }
+
+    {
+        auto rgb = img.to_rgba8_premultiplied();
+        REQUIRE(rgb.has_value());
+        REQUIRE(rgb->width() == 1);
+        REQUIRE(rgb->height() == 1);
+        REQUIRE(*rgb->begin() == Rgba8Pixel { 255, 0, 0, 255 });
+    }
+}
+
 TEST_CASE("SharedVector")
 {
     using namespace slint;