Load font file in FontConfig when using Pango

We need the text rendering capabilities of Pango (i.e. kerning), so we built node-canvas with 'with_pango' set to true. Problem is, that we use a custom TrueType font and it seems that node-canvas doesn't load it properly when using Pango.

The proposed change fixes the font loading step by calling FontConfig API.

Sample code:

    var canvas = new Canvas(400,40);
    var ctx = canvas.getContext('2d');
    var Font = Canvas.Font;
    
    var myFont = new Font('MyFont', '../fonts/pip.ttf');
    myFont.addFace('../fonts/pipBold.ttf', 'bold');
    
    // Do not call addFont when using Pango
    //ctx.addFont(myFont);
    
    ctx.font = "30px MyFont";
    ctx.fillText("kerning enabled AVAVAVA", 2, 32);