st-theme-node-drawing: Render all shadow sections with thickness > 0

In some styles the top or left sections of box-shadows would go missing even though they had non-zero thickness. This was because the inner local coordinates could become very slightly negative and were then incorrectly judged to be invisible.

Negative coordinates should not be ignored because that's where the entire top/left sections of shadow are meant to exist. It's only the sections of shadow with zero thickness that we should skip.

Before	After

Related to (but not a complete fix for) #3068 (closed) and #1090.