tdf#71034: [OS X] Ugly (too heavy) fake bold

Instead of using a hardcoded stroking value, adopt the formula used by
AOO which seems to give better results.

Change-Id: I87d55e1fadccf4521c26cd60458d968fcd8a2386
Reviewed-on: https://gerrit.libreoffice.org/15553Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>

vcl/quartz/ctfonts.cxx

@@ -80,7 +80,7 @@ CoreTextStyle::CoreTextStyle( const FontSelectPattern& rFSD )
          ((mpFontData->GetWeight() < WEIGHT_SEMIBOLD) &&
           (mpFontData->GetWeight() != WEIGHT_DONTKNOW)) )
     {
-        int nStroke = -10.0;
+        int nStroke = -lrint((3.5F * pReqFont->GetWeight()) / mpFontData->GetWeight());
         CFNumberRef rStroke = CFNumberCreate(NULL, kCFNumberSInt32Type, &nStroke);
         CFDictionarySetValue(mpStyleDict, kCTStrokeWidthAttributeName, rStroke);
     }