ui: improve update_line_data()

[deanlee]

Continuing the optimization of update_line_data() after #25001 by eliminating all memory operations (3 allocations/deallocations and 2 copies).

Think this change is worth it since update_line_data() is the only ui.cc function needing memory operations and is often called in loops.

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[sshane]

@adeebshihadeh this flipped the y coords for the path, causing the coloring to be inverted. Green ahead means red now!