Fixed|Models|Renderer: df_brightshadow2 and transparency

With "df_brightshadow2", the alpha multiplier of the submodel needs
to be applied even though the object has another kind of transparency
(MF_SHADOW).

IssueID #1725
Fixed|Renderer: Moving planes need decoration update on every frame

Whenever a plane's smoothed height changes, the surface needs to be
marked for decoration update so light decorations on the plane will
be correctly placed during the move.

IssueID #1705