fix: Performance improvement for scattergl with many points. Issue #7065 - #7301

The performance issues are always intriguing, and I tried to determine why the browser freezes when handling a large number of points. I believe the problem stems from two main factors.

The first factor is the hover effect, which triggers a function that calculates the distance between the mouse pointer and each point in the set. This process can certainly contribute to the performance problem, I believe the best improvement would be using a filter calculated by a window determined by the pointer coordinates with a delta, in this way, we can calculate the distance only for a subset. One possible improvement could be using squared distances for comparison, thereby avoiding the computation of square roots, similar to the method utilized by the K-d tree algorithm, if the plot allows it.

The second factor involves a loop in the part of the code I modified, where "newDistance" is created for every point. This results in excessive overhead for the garbage collector, causing the browser to freeze.

for(var i = 0; i < cd.length; i++) {
            var newDistance = distfn(cd[i]);
            if(newDistance <= pointData.distance) {
                pointData.index = i;
                pointData.distance = newDistance;
            }
        }

I moved the variable outside the cycle and used a precalculated value for the array length. I conducted tests in the development environments provided by Plotly using the test_dashboard tool.
The values represent average times measured in milliseconds..

The red line shows the performance for the variable inside, the blue line outside. The vertical axis (Y-axis) represents the time measured in milliseconds.

Of course, the performance depends on the engine and the environment in which it runs, but I think this conveys the idea of performance.

Please let me know if this can be the first step to improving performance.