message: Fix inconsistency in mix nodes default value
diff: <nodedef name="ND_mix_bsdf" node="mix" nodegroup="pbr" defaultinput="bg" doc="Mix two BSDF's according to an input mix amount.">
  <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]." />
  + <input name="mix" type="float" value="0.0" uimin="0.0" uimax="1.0" doc="Mixing weight, range [0, 1]." />
  <output name="out" type="BSDF" />
  </nodedef>
  <nodedef name="ND_mix_edf" node="mix" nodegroup="pbr" defaultinput="bg" doc="Mix two EDF's according to an input mix amount.">
  <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]." />
  + <input name="mix" type="float" value="0.0" uimin="0.0" uimax="1.0" doc="Mixing weight, range [0, 1]." />
  <output name="out" type="EDF" />
  </nodedef>
  <nodedef name="ND_mix_vdf" node="mix" nodegroup="pbr" defaultinput="bg" doc="Mix two VDF's according to an input mix amount.">
  <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]." />
  + <input name="mix" type="float" value="0.0" uimin="0.0" uimax="1.0" doc="Mixing weight, range [0, 1]." />
  <output name="out" type="VDF" />
  </nodedef>
  