Before issue #146, border-radius on row groups, rows, column groups, or columns don't apply to the background of each cell, yet the border-radius on the cell itself does.
After issue #146, the behaviors changed. In this patch, I tried to revert the behaviors of border-radius on table row groups, rows, column groups, or columns back to what happened before issue #146.
Also: Don't override GetBorderRadii in nsBCTableCellFrame.
This causes transparent stops to behave like "true transparent" instead
of "transparent black", even in RGBA space.
i.e.: the gradient will transition to a transparent version of the color
adjacent to the transparent color stop (on either side if not on the edge).