TEN: remove the sky texture size limitation

[Joey79100]

Tomb Editor stops level compilation if the sky texture selected isn't exactly 256*256px.
However, TombEngine doesn't have this requirement: it only has to be a square, and the dimensions have to be a multiple of 4*.

Tomb Editor should remove this artificial limitation for TombEngine, to allow for very crisp clouds textures.

* I removed this check on my end and tested random resolutions up to 8192*8192 to verify this, but haven't actually checked the TombEngine code to see what is the actual requirement.
Here's the check:

Tomb-Editor/TombLib/TombLib/LevelData/LevelSettings.cs

Line 636 in 1060fb0

if (image.Width != 256 || image.Height != 256)

(come to think of it, maybe the same is true for original engine, except the max resolution is 256*256?)