Adding texture array handling/previewing

[Bmarquez1997]

Currently, a few textures in the Fortnite files are stored as UTexture2DArray objects, which pack multiple images into a single image with multiple layers. For example, TA_Atlas_Figure_EyeAndLash_Char01 has 20 different textures packed into that single object.

This PR adds handling for UTexture2DArray objects, and allows the user to view and save all layers of an image instead of just the first layer.

Depends on CUE4Parse PR 145