Added support for background-origin: content-box, fixed background-origin related background sizes

[matthias-christen]

Summary

This PR fixes/implements the following bugs/features

• Bug: When using background-origin and background-size: cover/contain, the background size was not calculated correctly.
• Feature: Added support for background-origin: content-box

Explain the motivation for making this change. What existing problem does the pull request solve?

Add support for background-origin: content-box and fix for problem related to background image size with background-origin set to padding-box or content-box and background-size set to cover/contain.

Test plan (required)

Added tests/reftests/background/origin.html.

Code formatting

Please make sure that code adheres to the project code formatting. Running npm run format will automatically format your code correctly.

[niklasvh]

Thanks for the PR! I fixed the test and refactored the code a bit.

[matthias-christen]

Thanks for the great project and for merging the PR so quickly :)