allow text atlas to be exported and replaced to support batching

[fisherevans]

text.NewAtlas does the work of rendering a font onto an Atlas sprite to optimize rendering. But this prevents the text package from drawing onto a canvas using a shared Atlas.

• Atlas.PictureDataCopy allows you to make a copy of the atlas image, which you can then draw onto another shared atlas.
• Atlas.CloneWithPictureData allows you to make a new text.Atlas, referencing a shared atlas.

With both of these functions, you can draw text to a pixel.Batch, sharing an atlas with other fonts, sprites, or any other picture data.

Usage would look something like this:

// normal text atlas
textOnlyAtlas := text.Atlas7x13
textAtlasImg := textOnlyAtlas.PictureDataCopy().Image()

// draw the text atlas to a large, shared picture atlas
x, y := 100, 700
textAtlasPlacement := image.Rect(x, y, x+textAtlasImg.Bounds().Dx(), y+textAtlasImg.Bounds().Dx())
sharedAtlasImage := image.NewRGBA(image.Rect(0, 0, 1000, 1000))
draw.Draw(sharedAtlasImage, textAtlasPlacement, textAtlasImg, sharedAtlasImage.Bounds().Min, draw.Over)
sharedAtlasPiture := pixel.PictureDataFromImage(sharedAtlasImage)
// ... add more sprites to the shared atlas

// create a new text drawer with the shared atlas
pixelPlacement := convertFromImageToPixelRect(textAtlasPlacement)
sharedTextAtlas := textOnlyAtlas.CloneWithPictureData(sharedAtlasPiture, pixelPlacement)
textDrawer := text.New(pixel.ZV, sharedTextAtlas)

// draw to a batch
batch := pixel.NewBatch(nil, sharedAtlasPiture)
textDrawer.WriteString("Hello, world!")
textDrawer.Draw(batch, pixel.IM)
// ... draw other things to the batch that aren't text, that use the same source atlas picture

batch.Draw(canvas)