fix 2D assumption in point-based convert_args for lines

[asinghvi17]

Description

Fixes # (issue)

Allows plotting 3D polygons via lines and scatter, previously not possible because a Point2 was injected. This PR removes the 2D assumption, instead using a Point{N, float_type(T)}(NaN).

Type of change

Delete options that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.

[MakieBot]

Benchmark Results

SHA: 8648a5d2fab49c35da98b127b9ff68d9772a15b7

Warning

These results are subject to substantial noise because GitHub's CI runs on shared machines that are not ideally suited for benchmarking.

[asinghvi17]

WGLMakie failures seem unrelated.