Fix height paragraph box height calculation

The insets were removed from the "overhead" when they should have been
added. This was asking the text for its preferred height if it had
(width + insets) space, rather than (width - insets).

Because of this, some paragraphs give a preferred height of one line
less than they should (because they wouldn't need to wrap given the
extra space that's actually occupied by the insets).

Closes #809

richtextfx/src/main/java/org/fxmisc/richtext/ParagraphBox.java

@@ -228,7 +228,7 @@ protected double computePrefWidth(double ignoredHeight) {
     @Override
     protected double computePrefHeight(double width) {
         Insets insets = getInsets();
-        double overhead = getGraphicPrefWidth() - insets.getLeft() - insets.getRight();
+        double overhead = getGraphicPrefWidth() + insets.getLeft() + insets.getRight();
         return text.prefHeight(width - overhead) + insets.getTop() + insets.getBottom();
     }