PR summary

Agg already has RGB resampling with output to RGBA builtin, so we just need to correctly wire up the corresponding templates. With this RGB resampling mode, we save the extra copy from RGB to RGBA in NumPy land that was required for the previous always-RGBA resampling.

With the example from #29434, this saves 800MB of peak memory usage, which actually works out to two copies of 10000x10000x4 bytes; I'm not sure where the second copy comes from.

While this does pass tests, I think there may be some room for further improvement, as the input to the C++ code is A[..., :3], which should be a discontiguous view, but then the pybind11 code forces it to be contiguous, which would make another copy. There are some offset and step settings in Agg that I think we can use to avoid this copy as well, but it might require extra template expansions.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• [n/a] New Features and API Changes are noted with a directive and release note
• [n/a] Documentation complies with general and docstring guidelines