Make padding applied as swap chain panel's margin

[Summon528]

Summary of the Pull Request

Make padding applied as swap chain panel's margin

References

PR Checklist

• Closes Scrollbar is inset by padding and probably shouldn't be #1365 and potential issues with Implement background image over acrylic or solid color #1107
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• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Detailed Description of the Pull Request / Additional comments

I know that this PR is trying to do the same thing as #1672. However since that one is unlikely to be merged in a short time and the issue will cause background image to be padded once #1107 is merged, I present my slightly more straight forward solution which is just set margins to swap chain panel. Hope this one gets merge sooner but we can switch to #1672's solution later if it is preferred.
Note that this PR does NOT fixx #1367 as #1672 does.

Validation Steps Performed

Set padding and observe that the scroll bar stay in the same place

[zadjii-msft]

This is a perfectly fine change, but like you mention, is pretty much the same thing as #1672. I'm not sure what the benefits of that PR are over this one. This one seems like the more elegant, simpler solution, without need to muck with the renderer. Though, #1672 doesn't seem terribly complicated, and I'm pretty okay with it. I think I'd rather have @miniksa weigh in on whether the other PR makes sense. If he thinks the other is too risky or might take too long to review, I'm fine shipping this one.

[miniksa]

I like this one better because it doesn't mess with any math calculations in the DX renderer. It just applies the setting to a different element.

I just don't want the DX renderer to move too much before I get a big chance to put a concerted effort back into it, especially by adding mathematical deltas for offsets because I've burned myself that way before with complexity in rendering.

[zadjii-msft]

Makes sense to me.

[ghost]

🎉Windows Terminal Preview v0.3.2142.0 has been released which incorporates this pull request.:tada:

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