message: Fixed flip of fg and bg inputs on mix nodes.
diff: -->
  <nodedef name="ND_mix_bsdf" node="mix" type="BSDF" nodegroup="pbr" defaultinput="bg"
  doc="Mix two BSDF's according to an input mix amount.">
  - <input name="bg" type="BSDF" value="" doc="First BSDF."/>
  - <input name="fg" type="BSDF" value="" doc="Second BSDF."/>
  + <input name="fg" type="BSDF" value=""/>
  + <input name="bg" type="BSDF" value=""/>
  <input name="mix" type="float" value="1.0" uimin="0.0" uimax="1.0" doc="Mixing weight, range [0, 1]."/>
  </nodedef>
  <nodedef name="ND_mix_edf" node="mix" type="EDF" nodegroup="pbr" defaultinput="bg"
  doc="Mix two EDF's according to an input mix amount.">
  - <input name="bg" type="EDF" value="" doc="First EDF."/>
  - <input name="fg" type="EDF" value="" doc="Second EDF."/>
  + <input name="fg" type="EDF" value=""/>
  + <input name="bg" type="EDF" value=""/>
  <input name="mix" type="float" value="1.0" uimin="0.0" uimax="1.0" doc="Mixing weight, range [0, 1]."/>
  </nodedef>
  <nodedef name="ND_mix_vdf" node="mix" type="VDF" nodegroup="pbr" defaultinput="bg"
  doc="Mix two VDF's according to an input mix amount.">
  - <input name="bg" type="VDF" value="" doc="First VDF."/>
  - <input name="fg" type="VDF" value="" doc="Second VDF."/>
  + <input name="fg" type="VDF" value=""/>
  + <input name="bg" type="VDF" value=""/>
  <input name="mix" type="float" value="1.0" uimin="0.0" uimax="1.0" doc="Mixing weight, range [0, 1]."/>
  </nodedef>
  