Draft: crossbuild gstreamer using meson --cross-file
Marked as draft because it includes patched imath. Patching imath is not necessary anymore after the version in experimental gets uploaded to unstable. See bugs #1060843 and #1061327
We don't want to build our own imath because we don't want to ship patched imath in the MNT repos.
Currently, uploading imath from experimental to unstable is blocked by the 64bit time_t transition and autopkgtest regressions on armel and armhf.