Image padding corrections

[bijington]

• padding is applied on top of the original image size so that any padding above 1 results in actual padding being applied around the image Fixes #240

[bijington]

Result in this beauty being generated using the following:

var image = new OutputImage
{
    Height = 0,
    Width = 0,
    InputFile = Path.Combine(TestConstants.ImageDirectory, "logo.svg"),
    RequiresBackgroundColor = false,
    Scale = 1.0,
    ShouldBeVisible = true,
    Watermark = null,
    BackgroundColor = "Red",
    PaddingColor = "Yellow",
    PaddingFactor = 1.5
};