pszHost should just be set to pszTargetName if pszTargetName doesn't
contain any other information.

wininet: Retrieve the maximum token length from the SSP and use a buffer of that length in calls to InitializeSecurityContextW.

Otherwise, InitializeSecurityContextW could run out of space with our
small, fixed buffer and fail.

When using SSPI, pass in a NULL identity in this case so that the SSP
can try to use cached credentials.

wininet: Make the reference count of WININETHANDLEHEADER thread-safe by using InterlockedIncrement/InterlockedDecrement.

Test if a P8 texture gets properly updated if there was a palette change while 
this texture wasn't bound to any stage.