common Scene: return FailedAllocation if it really failed at memory allocation.

Due to change of f0ecc67, the return type of reserve() is changed to bool,
which can return the result for the fail case.

src/lib/tvgScene.cpp

@@ -56,7 +56,7 @@ Result Scene::push(unique_ptr<Paint> paint) noexcept
 
 Result Scene::reserve(uint32_t size) noexcept
 {
-    pImpl->paints.reserve(size);
+    if (!pImpl->paints.reserve(size)) return Result::FailedAllocation;
 
     return Result::Success;
 }