Fix CairoMakie heatmap offsets

[ffreyer]

Description

Fixes #4053

We extend heatmap cells by half a pixel in every direction when alpha == 1 to avoid gaps opening. This shifts the borders by half a pixel. I removed the padding for the side that covers other cells which fixes things for me. The other 0.5 padding is still there so this should still close gaps.

Type of change

• Bug fix (non-breaking change which fixes an issue)

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: f6ef3c8de29c822504f9e30b6168650f8e9a1495

Warning

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

[ffreyer]

Widened the pad on the hidden side to a full pixel/unit since some 3D tests show gaps: (And fail because of it)

[ffreyer]

Tests still fail because cells shifted (es expected). Heatmap 3D might be slightly worse in terms of coverage/gaps now, hard to tell. You can see the cell size being more accurate there though