Remove X3206 warnings generated by some shaders

Beginning in the August 2009 release of the DirectX SDK, the HLSL compiler will warn when an implicit truncation of a vector type occurs. Example:

shaders/common/scatterSkyV.hlsl (line 161)
replace:

Out.pos = In.position;

with:

Out.pos = In.position.xyz;

shaders/common/imposterV.hlsl (line 214)
replace:

finalPt.xyz = In.center + ( upVec * halfSize ) + ( rightVec * halfSize );

with:

finalPt.xyz = In.center.xyz + ( upVec * halfSize ) + ( rightVec * halfSize );

This fix is applicable to a variety of shaders. Not knowing if there's already a thread about it, I thought I'd post the same.