LibWeb: Add basic support for HTMLCanvasElement.toDataURL() :^)

This allows you to serialize a <canvas> element's bitmap into a
data: URI. Pretty neat! :^)

Userland/Libraries/LibWeb/HTML/HTMLCanvasElement.cpp

@@ -24,8 +24,10 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/Base64.h>
 #include <AK/Checked.h>
 #include <LibGfx/Bitmap.h>
+#include <LibGfx/PNGWriter.h>
 #include <LibWeb/CSS/StyleResolver.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/HTML/CanvasRenderingContext2D.h>
@@ -102,4 +104,14 @@ bool HTMLCanvasElement::create_bitmap()
     return m_bitmap;
 }
 
+String HTMLCanvasElement::to_data_url(const String& type, [[maybe_unused]] Optional<double> quality) const
+{
+    if (!m_bitmap)
+        return {};
+    if (type != "image/png")
+        return {};
+    auto encoded_bitmap = Gfx::PNGWriter::encode(*m_bitmap);
+    return URL::create_with_data(type, encode_base64(encoded_bitmap), true).to_string();
+}
+
 }