Fix C++ API observers with sparse components by filling out iterator field flags in flecs_uni_observer_invoke

[Megafunk]

I noticed C++ observers with sparse components cause this assert to fire:

fatal: flecs.c: 10947: assert: !(idr->flags & EcsIdIsSparse) use ecs_field_at to access fields for sparse components (INVALID_OPERATION)

A simple repro:

#include "flecs.h"
#include <iostream>

int main(int, char *[])
{
    struct Position {
        double x, y;
    };
    
    flecs::world ecs;
    ecs.component<Position>().add(flecs::Sparse);

    ecs.observer<Position>().event(flecs::OnSet).each([](flecs::entity e, Position& Pos)
    {
       std::cout << " - Position: {" << Pos.x << ", " << Pos.y << "}\n";
    });

    for (int i = 0; i < 4; ++i)
    {
        flecs::entity MeshEntity1 = ecs.entity();
        MeshEntity1.set<Position>({1,2});
    }
    
    ecs.progress();
}

After some messing around I found that each_delegate::invoke(ecs_iter_t *iter) seems to expect ref_fields to be set to determine if it uses terms.populate rather than terms.populate_self. I am not sure if this is the correct fix as this is the first time I've really done anything in flecs internals.

I think it would be useful to run the test cases with different combinations of sparse and non-sparse components to catch things like this ahead of time for commonly used API things.

[SanderMertens]

Nice! Can you add a test case for the issue as well?

[Megafunk]

I added a single test for sparse on set observers based on the others in the file

[SanderMertens]

Thanks for the PR!