Make UDMF scale compatible with ZDoom's spec

In the ZDoom UDMF specification, scale is supposed to be an alias for setting both xscale and yscale at the same time. These scale values are purely visual, meaning that it is functionally equivalent to SRB2's sprite scale variables.

This means when SRB2 uses the scale variable for object scale, UZB is a bit awkward with how it's handled, since it was designed for two fields, and doesn't copy or reload them properly. I think it's overall better to be compatible with how ZDoom uses these variables the variables and make a separate variable, mobjscale, for SRB2's own feature.

This change will break UDMF maps that use the scale feature, unfortunately, but that's why this needs taken care of ASAP.