consistent font line-spacing for Label

changes multi-line Label line-spacing basis from per-label max-glyph-height to font-size

Author: nmschulte

Source/Scene/LabelCollection.js

@@ -407,7 +407,7 @@ function repositionAllGlyphs(label) {
     horizontalOrigin,
     backgroundPadding
   );
-  var lineSpacing = label._lineHeight / label._relativeSize || defaultLineSpacingPercent * maxLineHeight;
+  var lineSpacing = (defined(label._lineHeight) ? label._lineHeight : (defaultLineSpacingPercent * label._fontSize)) / label._relativeSize;
   var otherLinesHeight = lineSpacing * (numberOfLines - 1);
   var totalLineWidth = maxLineWidth;
   var totalLineHeight = maxLineHeight + otherLinesHeight;