Support summary containing multiple where reductions with the same selector

[ianthomas23]

Fixes #1270.

Previously, a summary reduction containing multiple where reductions that use the same selector gave an error. With this PR they work as expected.

Example of what previously failed but now works:

canvas.line(...,
    agg=ds.summary(
        name1=ds.where(ds.max("value"), "other"),
        name2=ds.where(ds.max("value")),
    )
)

[codecov]

Codecov Report

Merging #1271 (ac5e125) into main (ea163e9) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1271      +/-   ##
==========================================
+ Coverage   83.51%   83.53%   +0.01%     
==========================================
  Files          35       35              
  Lines        9117     9128      +11     
==========================================
+ Hits         7614     7625      +11     
  Misses       1503     1503              

Files Changed	Coverage Δ
datashader/compiler.py	89.79% <100.00%> (+0.39%)	⬆️

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