Fix duplicate backgrounds when using horizontal and vertical editor split

[Katsute]

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Changes Made

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• Closes Vertical editor split is not randomized #68

Very complicated css logic:

First, handle horizontal splits only:

#workbench\\\\.parts\\\\.editor

:not(.split-view-container) // prevent styling horizontal + vertical combo

.split-view-container > .split-view-view:nth-child(\${len}n+\${j+1})

> .editor-group-container::after

for i = 0; i < len; i++

i	0	1	2	3
0	0	1	2	3

For col i, image index is simply i

Then, handle horizontal and vertical splits:

#workbench\\\\.parts\\\\.editor

.split-view-container > .split-view-view:nth-child(\${len}n+\${i+1})

.split-view-container > .split-view-view:nth-child(\${len}n+\${j+1})

> .editor-group-container::after

for i, j = 0; i, j < len; i, j++

i \ j	0	1	2	3
0	0	1	2	3
1	1	2	3	0
2	2	3	0	1
3	3	0	1	2

For col i, row j, derive image index using (i+j)%len

[Katsute]

Uninstall doesn't seem to work properly, possible caused by longer write.

[Katsute]

Instead of having a separate css statement for each i, j; add each css selector to an array based on which background image it's for.

Reduces the amount of css background image references from i*j to i. Seems to also fix the uninstall issue since the redundant css has been removed.

[ghost]

@Katsute I have approved this pull request