use scatter add in test of drawing full scenes

[ismael-mendoza]

@beckermr I add the scatter add approach to the test that draws full scenes which replaces the old scan approach

[beckermr]

LGTM and thank you!

[codspeed]

Merging this PR will degrade performance by 54.56%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 4 regressed benchmarks
✅ 34 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	test_benchmark_spergel_init[run]	40.7 µs	158.7 µs	-74.33%
❌	WallTime	test_benchmark_spergel_calcfluxrad[run]	123.7 µs	299.4 µs	-58.7%
❌	Simulation	test_benchmark_spergel_init[run]	1.3 ms	2 ms	-37.08%
❌	Simulation	test_benchmark_spergel_calcfluxrad[run]	1.7 ms	2.6 ms	-36.1%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing scatter-add-test (764e5f2) with main (ae9598a)}