Interpolate Shadows when Scaling Mobjs

When a mobj is scaling up or down, the mobj's scale is interpolated smoothly, but its shadow's scale is uninterpolated. This is because that a shadow is rendered with the mobj's radius (not scale!) multiplied by its shadowscale, neither of which are interpolated as radius "doesn't" relate to rendering and shadowscale is usually set as an instant change (e.g. setting it to 0 when bopping enemies). (Scaling a mobj changes it's radius proportionately.)

This can be fixed in two ways:
- Interpolate a mobj's radius when rendering, and make functions to either "set" or "interpolate" changes to radius.
- Multiply a shadow's scale by the proportion between its mobj's interpolated scale and uninterpolated scale.
This merge request opts for the latter.

This merge request does not interpolate mobjs' shadowscale; changing shadowscale is still instant. It's only the shadow's natural scaling from changing a mobj's scale that gets interpolated.

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TL;DR: Fixed an interpolation artefact