(#373): Render gradients with userSpaceOnUse even for an empty bounding box
In the test, a horizontal or vertical line was not being styled with a gradient, because those lines have an empty bounding box. However, the gradient in question had gradientUnits="userSpaceOnUse", not "objectBoundingBox". So, we don't need the object's bounding box at all, and the lines can use the gradient in the current coordinate system.
Fixes #373 (closed)