opengl: fix inv.scale values are always integer - cid#1352479

Force cast to double to avoid integer division - which gives a
wrong inverse scale value.

Change-Id: I0135e44ef07f3915619f9dfead9aadf50fc03685

vcl/opengl/gdiimpl.cxx

@@ -1384,8 +1384,8 @@ void OpenGLSalGraphicsImpl::DrawTransformedTexture(
     if( nDestHeight == 0 || nDestWidth == 0 )
         return;
 
-    const double ixscale = rTexture.GetWidth()  / nDestWidth;
-    const double iyscale = rTexture.GetHeight() / nDestHeight;
+    const double ixscale = rTexture.GetWidth()  / double(nDestWidth);
+    const double iyscale = rTexture.GetHeight() / double(nDestHeight);
 
     bool areaScaling = false;
     bool fastAreaScaling = false;