fix: do not draw from pixel for InterpolatedImage photon shooting

[beckermr]

This PR fixes a bug for photon shooting for interpolated images. I had coded it up to draw uniformly from the pixel of the interpolated image and then convolve in the interpolant kernel. That is wrong and instead it should simply treat the original image as a set of delta functions.

Working on comment https://github.com/GalSim-developers/GalSim-for-JAX-GalSim-Testing/pull/1/changes#r3028386824

[codspeed-hq]

Merging this PR will improve performance by 33.67%

⚠️ 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

⚡ 1 improved benchmark
✅ 35 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	test_benchmarks_lanczos_interp[xval-no_conserve_dc-run]	1,107.6 µs	828.7 µs	+33.67%

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_{Comparing test-thresh-1 (ed69e21) with main (58bb47e)}

[beckermr]

cc @ismael-mendoza