Always draw span borders, even if they occupy the entire span's width

Not having borders around some spans looks inconsistent and like they're
not a proper part of the timeline. We'd rather cover up the handful of
colored pixels with borders than not have borders.

This also fixes a bug when a borderless span was left to a bordered
span, where the right span was missing its left border, thinking the
span to its left had a right border.

cmd/gotraceui/timeline.go

@@ -888,7 +888,7 @@ func (track *Track) Layout(
 			highlightedSpans = append(highlightedSpans, clip.FRect{Min: minP, Max: maxP})
 		}
 
-		if dspSpans.Len() != 0 && dspSpans.AtPtr(0).State != statePlaceholder && endPx-startPx > float32(minSpanWidth) {
+		if dspSpans.Len() != 0 && dspSpans.AtPtr(0).State != statePlaceholder {
 			off := float32(spanBorderWidth)
 			leftOff := off
 			rightOff := -off