Example enable-background01
This test case shows five pictures which illustrate the rules
for background image processing.
This filter discards the SourceGraphic, if any, and just produces
a result consisting of the BackgroundImage shifted down 125 units
and then blurred.
This filter takes the BackgroundImage, shifts it down 125 units, blurs it,
and then renders the SourceGraphic on top of the shifted/blurred background.
The first picture is our reference graphic without filters.
The second adds an empty 'g' element which invokes ShiftBGAndBlur.
The third invokes ShiftBGAndBlur on the inner group.
The fourth invokes ShiftBGAndBlur on the triangle.
The fifth invokes ShiftBGAndBlur_WithSourceGraphic on the triangle.