Gaussian blur filter differs if the filter bounding box is smaller than the source

The result of the baseline test "boxGauss.svg" differs from a rendering by chrome and firefox. This test combines various filter boxes with transformations.

The reason is that we trim the source image to the filter box, whereas this seems not to be done in Chrome.

This should have little effect on real world examples because artists usually try to contain the item within the filter rules. This might require to rethink and rewrite the handling of transformations in the blur filter.