Move the checks for float texture formats into the correct location.
This allows some Direct3D sample applications which check for any of
the float formats together with D3DUSAGE_RENDERTARGET to start.

wined3d: Ensure all states affected by a change in render_offscreen are marked dirty in ActivateContext.

oleaut32: Clear the input pvar structure in VARIANT_UserUnmarshal to free the data instead of just initializing.