Auto merge of #29368 - mrobinson:conditionally_use_certain_gstreamer_libraries, r=mukilan

Conditionally use certain GStreamer plugins

In old versions of GStreamer gstvideoconvertscale was split into two plugins, gstvideoscale and gstvideoconvert. This change makes the use of these three plugins conditional on their existence and has Windows use the two plugins from the older version of GStreamer. In addition, it starts to clean up the gstreamer.py file which is responsible for managing GStreamer libraries and plugins. This is the first change in a series of changes that seek to clean up the way these libraries are managed during the build.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they should change behavior except for old GStreamer on Mac.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

0 files changed, 0 insertions, 0 deletions