RebuildRenderData is still not safe during cook time

https://answers.unrealengine.com/questions/215370/some-sprites-tiling-into-tiny-grid-in-packaged-ver.html

UTexture2D::ConditionalPostLoad doesn't wait for the render data to finish caching outside of the editor...

The RebuildRenderData is only getting called because the texture size is 0 in the first place, and it then mucks everything up using the fallback 1x1