Refactoring pass.

- Extract an enum for edges rather than using magic numbers for everything.
- Remove duplicated code between FoggedEdges and ShroudedEdges by hosting the visibility function into a delegate.
- Make minimap methods more readable.
- Tidy formatting.
- Make some fields readonly.
- Remove unused usings.
Sped up shroud rendering.

- Only update shroud within the visible screen area, rather than the whole map. This improves performance on larger maps significantly when scrolling around since large portions of the shroud do not need to be updated.
- Provide methods in Shroud to return delegates to check for explored/visibility for tiles within a certain region. This allows it to return more efficient delegates whenever the region is within the map bounds, or shroud/fog is disabled. In the typical case where the region is in bounds and shroud/fog is enabled, the fast check is almost twice as fast as the slow check.
- Use the Shroud delegate functions in shroud rendering, frozen actors, minimap rendering and resource layer areas to provide a speedup since these areas of code can often take advantage of the fact they perform checks within the map boundary.
- Cache current element in CellRegionEnumerator to prevent repeated work if the element is accessed more than once.
- Decrease the size of elements in some arrays in hopes of reducing memory needs and improving cache hits.
Make tiles sorting deterministic in all cases

This may fix issue #5916.

In any case, it's wanted because this kind of sort is "unstable".
According to the docs:

    "This implementation performs an unstable sort; that is, if two
    elements are equal, their order might not be preserved."
Fixed typo in HackAI building placement code
Merge pull request #5917 from pavlos256/deterministic-sort

Make tiles sorting deterministic in all cases
Merge pull request #5924 from Cr0s/patch-1

Fixed typo in HackyAI building placement code
Remove bitrotted tab source artwork.
Add opposing-faction tab art for aircraft and navy,
Fix incorrect soviet tab image coordinates.
Enable correct tab images for helicopters and aircraft.
Fix bogus inconsistencies in the ra tab images:

- Soviet-normal tab glyphs were off by 1px.
- Allied-selected tabs had random semi-transparent pixels.
- Most tabs had a discoloured pixel near the left edge.
Sort Changes
Merge pull request #5927 from MustaphaTR/ra-building-sort

Fixes #5914
Merge pull request #5926 from pchote/ra-tabs

Fixes #5915
Explosive Oil Derricks
Changed some tiles types
Update fort-lonestar.lua
Merge pull request #5938 from abcdefg30/fortlonestar

Fixed a bug in Fort Lonestar
disable tech properly
Merge pull request #5940 from Mailaender/fort-lonestar-tech-disable

Fixed unbuildable high-tech on Fort Lonestar