[patch] Improve PowerLines test performance

[gaben]

While working on #24851, I discovered some inefficiencies. In my tests, I managed to make the test run approximately 8 times faster.

I'll work on the patch to make sure nothing broke, because I'm a bit suspicious.

The patch will follow soon. Ping me if I forget.

[gaben]

Something like this. Avoiding the per-node evaluation of findCrossings() gives an 8x performance increase on my test data with the same output.

There are also some memory allocation improvements.

[stoecker]

Apply?

[gaben]

In 19658/josm:

fix #24876 - improve PowerLines test performance

The test looked up the water crossings of the whole way once for every node of the way, although the result only depends on the way.
On a large power line dataset (~500k primitives), a full run of the test goes from ~8.2s to ~0.9s, with identical results.