msdkenc: fix leaks on windows

commit ce2a94f5:

msdk: Use gst_clear_object()
`gst_object_replace()` is not supposed to be used for unreffing and
NULLing objects.

commit 2c4c9828:

msdk: Fix warning about unused variable on Windows

commit 4384dbd8:

msdk: Reorganize context preparation code
Split it out into a separate function with early exits to make the
flow clearer, and document what the function is doing clearly.
No functional changes.

commit b64693fe:

msdk: Fix increasing memory usage in dynamic pipelines
Our context is non-persistent, and we propagate it throughout the
pipeline. This means that if we try to reuse any gstmsdk element by
removing it from the pipeline and then re-adding it, we'll clone the
mfxSession and create a new gstmsdk context as a child of the old one
inside `gst_msdk_context_new_with_parent()`.

Normally this only allocates a few KB inside the driver, but on
Windows it seems to allocate tens of MBs which leads to linearly
increasing memory usage for each PLAYING->NULL->PLAYING state cycle
for the process. The contexts will only be freed when the pipeline
itself goes to `NULL`, which would defeat the purpose of dynamic
pipelines.

Essentially, we need to optimize the case in which the element is
removed from the pipeline and re-added and the same context is re-set
on it. To detect that case, we set the context on `old_context`, and
compare it to the new one when preparing the context. If they're the
same, we don't need to do anything.

Fixes #946 (closed)