All textures use relative path except embedded textures, this is a fix for it

[muxanickms]

FBXConverter::GetTexturePath returns RelativeFilename() for all textures (this is stored in the material $tex.name), but FBXConverter::ConvertVideo returns FileName() as a texture name, so it's very hard to match both strings in run-time to figure out which embedded texture it is.
I propose to use relative path everywhere, that helps a lot.

[kimkulling]

Merged, thanks a lot for the contribution!