Create FMeshSprite

[MattRix]

FMeshSprite (or maybe FMeshNode? FCustomSprite?) should be an easy way to create a sprite made out of a custom 2D mesh by just passing in some mesh data (element, verts, uvs, colors), rather than having to create an FFacetNode.

Maybe FElementMeshSprite and FMeshSprite? - one would extend FFacetNodeBase and one would extend FFacetNode? (basically defining whether the uv coords are relative to an element, or relative to an entire texture)

Leaning towards "FCustomSprite" and "FCustomElementSprite"