draw: fix crash caused by Scale by Copy shortcut

When DstMask is not nil, this shortcut causes stack overflow because
Copy function in turn will call Scale with same dr and sr.

Fixes golang/go#23107

Change-Id: I8ccadbd9b7f16363ac17b6114308527d6fa9456e
Reviewed-on: https://go-review.googlesource.com/83537
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Run-TryBot: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
3 files changed
tree: f2f228bad8cd605fa085b0b8293448ef36857741
  1. bmp/
  2. cmd/
  3. colornames/
  4. draw/
  5. example/
  6. font/
  7. math/
  8. riff/
  9. testdata/
  10. tiff/
  11. vector/
  12. vp8/
  13. vp8l/
  14. webp/
  15. .gitattributes
  16. .gitignore
  17. AUTHORS
  18. codereview.cfg
  19. CONTRIBUTING.md
  20. CONTRIBUTORS
  21. LICENSE
  22. PATENTS
  23. README.md
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