Imagefreeze add num-buffers property

The imagefreeze element can be handy for benchmarking downstream elements because it re-uses the same buffer memory and introduces less overhead compared to always creating new frames with videotestsrc.

However it's not possible to make imagefreeze send EOS when using gst-launch-1.0.

Add a num-buffers property to make it look more like a source in the above scenario.

Suggested by @ndufresne.

The first commit in the series seems required when building locally with gst-build to make the pre-commit hook happy, however the CI fails because of it https://gitlab.freedesktop.org/ao2/gst-plugins-good/-/jobs/169735