Fixed wrong empty line height

Inverse negative descent to calculate the height of an empty line correctly.

Also renamed lineGap to emptyLineHeight to avoid confusion with semantically different lineGap as defined in TrueType etc.

@@ -283,7 +283,7 @@ See https://github.com/DavBfr/dart_pdf/wiki/Fonts-Management
   double get descent;
 
   /// Height of an empty line
-  double get lineGap => ascent + descent;
+  double get emptyLineHeight => ascent + (-descent);
 
   /// Internal units per
   int get unitsPerEm;

@@ -1073,7 +1073,8 @@ class RichText extends Widget with SpanningWidget {
               if (spanCount > 0) {
                 offsetY += bottom - top;
               } else {
-                offsetY += font.lineGap * style.fontSize! * textScaleFactor;
+                offsetY +=
+                    font.emptyLineHeight * style.fontSize! * textScaleFactor;
               }
               top = 0;
               bottom = 0;