Fix rendering panic on Z resize

[Drulikar]

Description

When the quantity of Z levels are changed, and you aren't looking at the first Z, visibleLevel := r.bucket.Level(level) will be nil because the length of bucket levels is 1 instead of say 2. Let me know if there's a better way to flush out rendering work on a resize, but this is a more general solution that should be more reliable.

Fixes https://discord.com/channels/484170914754330625/485190298050363393/1476400437623259216 (Coderbus)

To reproduce:

• Create a new map with 3 Zs
• View Z 2
• Resize from 3 Zs to 2 Zs

Maybe it would address #386 and or #314 but I'm not able to reproduce them as described so I don't know.

Type of change

• Minor changes or tweaks (quality of life stuff)
• 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)
• This change requires a documentation update