Optimize Editor _init_grid()

[lawnjelly]

Use static LocalVectors instead of Vectors for temporaries.

Forward port of #92725

Notes

• Not profiled this but it should be significantly faster like [3.x] Faster editor grid #92725 because it avoids COW machinery (moo 🐮 ) and excessive allocations (the temporary vectors are static and will re-use memory, and only the final conversion to Vectors should cause allocations).
• The IDX indexing and bounds checking could also be avoided by using pointers, but I've left that alone for now as it slightly less safe to future bugs (anyone else feel free to change).

[clayjohn]

Looks great! Fewer memory allocations is always welcome

[akien-mga]

Thanks!