Disallow assigning external textures to Slate widgets

External textures cannot be directly assigned as a brush texture; you need to assign a material that samples from an external texture instead since it generates special shader code to access it.

1. Create UI Widget
2. Create Movie Player / media texture
3. Open UI Widget and assign media texture to a widget

This should not work; you need to create a slate material that uses a media texture and assign the material instead.