[KK] Add TextureUtils.CorrectColorSpaceForKoikatu for KK UI textures

[Lusiocc]

Description

KK (Unity 5.6, Linear) samples sRGB UI PNGs too dark. Add a public gamma-LUT fixer (KK-only, no-op elsewhere) and apply it to the InterfaceMaker skin.

Motivation and Context

KKs VE window and IMGUI windows in general were too dark.

How Has This Been Tested?

KK and HS2 satrted and checked if colors are correct.

Performance test:

Size	Pixels	Iters	M2 Color32+LUT
VE-small 32	1,024	10000	0.007 ms
medium 256	65,536	1000	0.345 ms
multi-MB 2048	4,194,304	100	19.999 ms
4K 4096	16,777,216	50	66.620 ms
8K 8192	67,108,864	10	286.915 ms

ms are average over the iterations NOT total time.
Performance test was made in a clean Unity project, since i tested multiple ways.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

[ManlyMarco]

The difference in KK is already handled by having a separate light texture that is pre-baked
If it's not close enough then it would be best to replace that texture instead, or if it's fixed at runtime then the light texture system should be scrapped instead.

The API is fine as it is for use by other plugins, but the name of the method does not indicate that it does not work in KK, either make it always work or name it something like CorrectColorForKoikatu.