Shadow mapping

[untodesu]

Description

Forward-rendering with shadow mapping.
The rendering API seems to already grant us such "features", the only real problem with that would be requirement to render the whole scene two (or more if we talk about PSSM) times.

Technical (optional)

There are already some juicy shadow filtering functions present in the SDK (in untouched Valve's code there's a couple, in this codebase only one but it's nice) and the only thing that is required is generating a shadow map and somehow hacking it into LightmappedGeneric code because it seems like the amount of shader constants is limited.