Stop mesh item from newing up drawable every frame

[joshuaskelly]

Summary

Due to a small logic error, we were newing up a mesh drawable every frame for mesh items. The fix was:

1. Split out the case for if meshToUse was null or not.
2. Put the check to see if our mesh needs updating inside the previous check.
3. Actually record the lastMeshFile and lastTextureFile values.
4. Use String.equals() for sting equality checks.