Growing memory usage on redraw

[timholy]

As reported on julia-users. I'm debugging this on the Gtk branch to take advantage of precompilation for fast reloading.

Here's a test script:

using Gtk.ShortNames, Base.Graphics, Cairo
using Images, TestImages

img = testimage("mandrill")
buf0 = Images.uint32color(img)

type MyObj
    surface::CairoSurface
end
mycanvas = MyObj(CairoImageSurface(buf0, Cairo.FORMAT_RGB24, flipxy = false))

c = @Canvas(size(buf0,1), size(buf0,2))
win = @Window(c, "Canvas")
draw(c) do widget
    ctx = getgc(c)
    buf = copy(buf0)
    rectangle(ctx, 0, 0, size(buf, 1), size(buf, 2))
    mycanvas.surface = CairoImageSurface(buf, Cairo.FORMAT_RGB24, flipxy = false)
    set_source(ctx, mycanvas.surface)
    fill(ctx)
    reveal(c, false)
end
showall(win)

# for i = 1:20; draw(c); end

If you call the draw loop at the end, memory grows, but gets freed by gc(). That doesn't seem to happen if instead you use ImageView:

imgc, imsl = view(img)
for i = 1:20; view(canvas(imgc), img); end

Not sure why or what's going wrong.