[BUG]: `scattergl` and `splom` hover ignores marker size for overlapping points

[camdecoster]

Description

If markers of different sizes overlap, scatter and scattergl show different hover labels. scatter includes the marker radius in the hover distance, and a smaller marker wins while the pointer is near its center (src/traces/scatter/hover.js). scattergl and splom measure only the distance to the marker center, so the marker drawn on top always wins. scatterpolargl inherits this through scattergl.

scattergl also uses a plain |dx| in x hovermode, but scatter changes the shape of dx inside the marker radius. That difference can rank close points differently.

This is a follow-up to #8077, which fixed ties between equal distances.

Steps to reproduce

• Open Plotly devtools
• Run this snippet in the browser devtools

Plotly.newPlot(gd, {
    data: [
        { type: 'scatter', name: 'A', mode: 'markers', marker: { size: 6 }, x: [1], y: [0] },
        { type: 'scatter', name: 'B', mode: 'markers', marker: { size: 20 }, x: [1], y: [0] }
    ],
    layout: { hovermode: 'closest' }
});

• Hover the center of the shared point
• Note that the label shows A
• Change both traces to type: 'scattergl' and hover again
• Note that the label shows B

Notes

• A fix probably means sharing the scoring functions in scatter/hover.js with scattergl and splom
• Each marker's radius can be taken from the sizes that scattergl/convert.js already computes
• Large traces search a spatial tree within hoverdistance of the pointer, so the search box also has to grow by the largest marker radius
• This would changed hover results for every WebGL plot with markers of different sizes, so we need to be careful about the blast radius