Make ImPlotPoint equal compatible

[volard]

Problem

Once I wanted to find_erase() from ImVector<ImPlotPoint> but then I got this:

Severity	Code	Description
Error	C2676	binary '==': 'T' does not define this operator or a conversion to a type acceptable to the predefined

Solution

Implement bool operator== by comparing equality of coordinates values

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There are probably a lot more improvements ImPlotPoint needs to have, but I found just this use-case so far (implot is the great library, thank you)