Rework dependencies stubs for FFmpeg

* Move the stub utils from the plugin to QtMM so that shared stubs
  could reuse them.
* Make the stubs as singletons instead of a static object to address
  initialization races and allow lazy loading (it's optional now).
* Decouple stubs definition: one file and singleton per stubbed lib.
  It gives us an advantage to create shared stubs separately,
  they are to be implemented in the next CR.

Task-number: QTBUG-120990
Pick-to: 6.7 6.5
Change-Id: I2158a49073b1e66d1bbfcb993b7746a4bd45048f
Reviewed-by: Jøger Hansegård <joger.hansegard@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

0 files changed, 0 insertions, 0 deletions