Don't allow nullptr in texels array params (unless using a transfer buffer).

Require all levels in writePixels to have a non-nullptr.
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Committed: https://skia.googlesource.com/skia/+/8ee78f31b2a29a5f76403755ea17bad9be74a3ec

Review URL: https://codereview.chromium.org/1765633002
6 files changed