LibAccelGfx+LibWeb: Store state of all stacking contexts in GPU painter

This change ensures that the GPU painting executor follows the pattern
of the CPU executor, where the state is stored for each stacking
context, but a painter is created only for those with opacity.

Fixes crashing on apple.com because now save() and restore() are called
on correct painters.

Userland/Libraries/LibAccelGfx/Painter.cpp

@@ -137,7 +137,7 @@ void main() {
 
 HashMap<u32, GL::Texture> s_immutable_bitmap_texture_cache;
 
-OwnPtr<Painter> Painter::create()
+NonnullOwnPtr<Painter> Painter::create()
 {
     auto& context = Context::the();
     return make<Painter>(context);