JpegEmbedder: Support Buffers that have data in their backing buffer is offset

[allenbenz-doma]

What?

This adds support when the jpeg image data in the backing buffer for a Buffer is offset from index 0.

Why?

Buffer.buffer isn't necessarily indexed the same as the Buffer.

Ran into this when some transparent optimization caused a single buffer to be used for multiple short hardcoded test image/non-image Buffers.
https://stackoverflow.com/a/50118846 talks about it, the gist of it that Buffer.from can call Buffer.allocUnsafe which may reuse memory from a pool, so image data can be stored in the middle of the buffer.

Someone could deliberately do this too with something like a sprite sheet.

How?

DataView already supports handling this and Uint8Array has the necessary offset/length fields, so added them to the DataView constructor.

Testing?

Wrote a new test, failed before change, passed after, existing tests pass, scope of the change is very small.
Only ran the nodejs and browser integration tests.

New Dependencies?

No

Screenshots

N/A

Suggested Reading?

https://nodejs.org/dist/latest-v10.x/docs/api/buffer.html#buffer_class_method_buffer_allocunsafe_size

Anything Else?

Checklist

• I read CONTRIBUTING.md.
• I read MAINTAINERSHIP.md#pull-requests.
• I added/updated unit tests for my changes.
• I added/updated integration tests for my changes.
• I ran the integration tests.
• I tested my changes in Node, Deno, and the browser.
• I viewed documents produced with my changes in Adobe Acrobat, Foxit Reader, Firefox, and Chrome.
• I added/updated doc comments for any new/modified public APIs.
• My changes work for both new and existing PDF files.
• I ran the linter on my changes.