Allow Slate thumbnail previews to match the aspect of the texture image

[d.york]

Currently all thumbnail previews of textures in slate or the standard material editor are locked to a square in the bitmap node. When working with numerous textures that have significantly different aspects they are distorted and become illegible in the preview.

Allowing for an option to allow the bitmap previews to match the bitmap aspect would increase productivity and simplify the process of updating/changing and texturing large scenes or numerous texture projects.