winebuild: Check if a given forward does exist in one of the imported dlls, fix a couple of problems detected.

This is so the filters can pass on and use timing, discontinuity, and other
information stored in the sample as needed.

While I was at it, simplified the symbol lookup for the debug options.

This is to allow StretchRect to pass the texture filter to WineD3D.
DirectDraw sets the texture filter to WINED3DTEXF_NONE, simmilar to all
other functions which do not need filtering.