allow text atlas to be exported and replaced to support batching

[fisherevans]

Problem

text.NewAtlas renders a font into a private *pixel.PictureData. Drawing text with pixel.Batch requires all sprites and glyphs to share the same backing picture, but there was no way to read the atlas image or create an atlas that references a larger shared picture.

Changes

• Atlas.PictureDataCopy — returns a deep copy of the atlas's pixel data so callers can blit it into a shared atlas image.
• Atlas.CloneWithPictureData(pic, frame) — returns a new Atlas backed by pic, with all glyph coordinates translated so they remain correct at their new location (frame) within the shared image.
• Atlas.pic narrowed from pixel.Picture to *pixel.PictureData (internal only - the exported Picture() method signature is unchanged).

Usage

// Build a font atlas and copy its pixels into a shared sprite sheet.
fontAtlas := text.NewAtlas(face, text.ASCII)
atlasCopy := fontAtlas.PictureDataCopy()

x, y := 100, 200
placement := image.Rect(x, y, x+int(atlasCopy.Bounds().W()), y+int(atlasCopy.Bounds().H()))
sharedImage := image.NewRGBA(image.Rect(0, 0, 1024, 1024))
draw.Draw(sharedImage, placement, atlasCopy.Image(), atlasCopy.Image().Bounds().Min, draw.Src)
// ... add more sprites to sharedImage

sharedPic := pixel.PictureDataFromImage(sharedImage)
frame := pixel.R(float64(x), float64(y), float64(placement.Max.X), float64(placement.Max.Y))
sharedAtlas := fontAtlas.CloneWithPictureData(sharedPic, frame)

// Now text and sprites can share a single pixel.Batch.
batch := pixel.NewBatch(nil, sharedPic)
textDrawer := text.New(pixel.ZV, sharedAtlas)
textDrawer.WriteString("hello")
textDrawer.Draw(batch, pixel.IM)
batch.Draw(win)

Test plan

• go build ./... unchanged.
• go test ./ext/text/... — new tests cover deep-copy semantics for PictureDataCopy, identity and translated placements for CloneWithPictureData, and the wrong-dimensions panic.