Mines are unhittable

[digama0]

The original code was using this.bbox.expandByScalar(-0.25); to decrease the size of mines (for reasons I was unable to determine), but the bbox is only 0.328 large in the x direction and this trims 0.25 from all sides, resulting in an inverted bbox. The z direction is even smaller, with an extent of only 0.05 or so, so on the assumption that the code was meant to reduce the size of the bomb by 50% it now shrinks the dimensions separately.

This code was originally added in 7074c8bb by @ngokevin , so maybe they can say what the intent of this change was.

The bug and the fix were tested on Oculus Quest 2 / Oculus Browser.