oleaut32: preliminary support for VT_DISPATCH as parameters to VarAdd (and other 2-operand operations)

[Alex Villací­s Lasso]

This is a preliminary patch to add support for VT_DISPATCH as parameters 
to VarAdd and other arithmetic operations with variants. The tests still 
"pass" in the sense that null VT_DISPATCH as issued by the tests are 
still rejected. I post this in the hope that it will be reviewed while I 
prepare a test case.

Changelog:
* Add support for receiving of VT_DISPATCH parameters in several 
operator functions by fetching the result value, then reissuing the same 
operation on result data.