Improve bucket preloading to GPU for weird scales / avoid flickering

[philippotto]

The oblique bucket picker uses the constant zDiff = 10 to load buckets to the GPU that are likely needed when going forward (or backwards) in the third dimension. This constant seems to be in unit space. If the dataset scale is very small (e.g., 1, 1, 1), buckets are preloaded for wrong section.

Steps to reproduce:

• set dataset scale to 1,1,1
• maximize XY viewport
• switch from z=31 to z=32 <-- flicker appears
• set webknossos.DEV.flags.bucketDebugging.visualizeBucketsOnGPU = true in the dev console, and check out the 3d viewport to see which buckets are loaded