LibWeb: Add ability to present LibGL framebuffer and add clearing

Author: https://github.com/Lubrsi
Commit: 076c9772a4
Pull-request: https://github.com/SerenityOS/serenity/pull/14184
Reviewed-by: https://github.com/Quaker762 ✅
Reviewed-by: https://github.com/gmta ✅
Reviewed-by: https://github.com/linusg ✅
Reviewed-by: https://github.com/sunverwerth

Userland/Libraries/LibWeb/Painting/CanvasPaintable.cpp

@@ -35,8 +35,11 @@ void CanvasPaintable::paint(PaintContext& context, PaintPhase phase) const
         if (!context.viewport_rect().intersects(enclosing_int_rect(absolute_rect())))
             return;
 
-        if (layout_box().dom_node().bitmap())
+        if (layout_box().dom_node().bitmap()) {
+            // FIXME: Remove this const_cast.
+            const_cast<HTML::HTMLCanvasElement&>(layout_box().dom_node()).present();
             context.painter().draw_scaled_bitmap(absolute_rect().to_rounded<int>(), *layout_box().dom_node().bitmap(), layout_box().dom_node().bitmap()->rect(), 1.0f, to_gfx_scaling_mode(computed_values().image_rendering()));
+        }
     }
 }