This allows widl to catch type redefinitions and report an error, like MIDL.
However, this does a better job than MIDL since it also reports the location
of the originial definition, like GCC.

widl: Increment the buffer offset by 8 for embedded pointers inside varying arrays in varying structures.

The 8 bytes are for the variance and offset and are added just before
the array, as seen in the marshalling code in rpcrt4. No offset needs
to be added for non-varying structures since the buffer mark is set
after the conformance in marshalled or unmarshalled.

This avoids sending uninitialised data on the wire.

The idea is to make setting depth attachments a bit more consistent
with set_render_target_fbo()/attach_surface_fbo(). I've also got an
upcoming patch in my tree that needs this.

1 bpp DDBs seem to always carry an implicit palette of black and white,
regardless of what the DC says. Based on a patch from Eric Pouech.

Windows always generates a black/white palette for 1 bit DIBs
generated from DDBs. Copying from the current palette only takes place
for color DDBs (not explicitly mentioned in MSDN), and only if depth
equal (MSDN states it, explicitly).

Adds VariantClear and removes unnecessary variant type assignment that may cause 
heap corruption.