[bug] GetStringBounds does not account for italics style

[kodawah]

Hi, I'm drawing some text on an image and color its background with ClearRect()

I noticed that when italics is set, the background and the image are incorrectly offset

see the picture, the string has some space before the text starts, and a section of the 't' is drawn outside the background

for both italics and normal style, GetStringBounds reports l:3.703125, t:-29.546875, r:204.656250, b:0.468750

here is the relevant portions of my code

    l, t, r, b := gc.GetStringBounds(str)
    strWidth  := int(math.Ceil(r)) - int(math.Ceil(l))
    strHeight := int(math.Ceil(t)) - int(math.Ceil(b))

    pointX := (input.Bounds().Dx() - strWidth) / 2
    pointY := (input.Bounds().Dy() - strHeight) / 2

    gc.SetFillColor(color.RGBA{255,255,255,255})
    gc.ClearRect(pointX, pointY, pointX + strWidth, pointY + strHeight)

    gc.SetFillColor(color.RGBA{0,0,0,255})
    gc.FillStringAt(str, float64(pointX), float64(pointY))

[llgcode]

We need to use the new freetype API to draw and compute string length. It may correct this issue.