Expose SetDirty

[microo8]

After this time, I think this #436 PR isn't going to be merged so I think there could be a simpler way.
What if we leave image tracking on the user, and just expose SetDirty(x, y int, dirty bool) method.

I'm working on a app that shows some images. I know the positions and regions these images occupy.
When the screen changes (I scroll down or something), I can calculate cells that need to be redrawn.
Just need the SetDirty function to force tcell to redraw the cells.
I was already thinking about some hacky ways, that I can set the backgroud color to a darker/lighter, then
screen.Show() and then right back to the original color. But that's not pretty.
Using screen.Sync() is too slow and the screen and all the images blink too much.

Would this be possible? Just expose SetDirty?

[microo8]

So I made it. I think it's hacky, but it's less work.
Before, there where just spaces in the area where the image was.
Now I Set the area to unicode's Braille Blank (or just the rune '\u2800')
I think it is the same width and tcell is forced to redraw the areas where the image was before,
because on the new frame, there are no Braille Blanks, just spaces.

so just:

var imageArea image.Rect = calculateImageArea()
for x := imageArea.Min.X; x < imageArea.Max.X; x++ {
    for y := imageArea.Min.Y; y < imageArea.Max.Y; y++ {
        s.SetContent(x+ctx.X, y+ctx.Y, '\u2800', nil, tcell.StyleDefault)
    }
}

and when there's no image just:

s.SetContent(x+ctx.X, y+ctx.Y, ' ', nil, tcell.StyleDefault)