Multi-button value editing would cancel when the entered number in the
active field was the same, even when the other values were not. E.g. a
multi-button with values [0.0, 1.0, 2.0] would not change it's value when
the user tried to drag (from the first field) downwards to multi-edit
and then type 0 (or any other value equivalent to 0).

This was because the optimization in rBeb06ccc32462 did not consider
multi-editing. The patch fixes this by comparing origvalue in the
uiButMultiState for all buttons in the block to the entered value.
If the values are the same, the operation is canceled (since we can be
sure that no update is required since no value changed).

An alternative to this patch would be to skip the optimization if
data->multi_data.has_mbuts is true. Although this would always
trigger an update, even if it might be unnecessary.