shlwapi/tests: The function pointers are global so initialize them on startup, before they are used.

Only specify the root to keep platform-specifics out of the individual 
DLL makefiles.

Add a new property of the shader backend which indicates whether the
shader backend is able to dirtify single constants rather than
dirtifying vshader and pshader constants as a whole. Depending on this
a different Set*ConstantF implementation is used which marks constants
dirty. The ARB shader backend uses this and marks constants clean
after uploading.