Potential GstContext regression

Potential GstContext regression

Since the refactoring of GstContext (commits
qc9fa2771b508e9aaeecc700e66e958190476f,
a7f5dc8b8af837f01782d1572379948ff62daab7,
690326f906dc82e41ea58b81cdb2e3e88b754,
d367dc1b0d4ecb37f4d27267e03d7bf0c6c06a6, and
82d158aed3f2e8545e1e7d35085085ff58f18) I am no longer able to get
a shared context for an element that is used twice in a pipeline.

I used the documentation and eglglessink as my reference for
implementing the GstContext logic.

As the code was tied to a hardware decoder, I have ported the
GstContext code to fakesink to show the problem. Using the old
API a single ExampleMgr instance is created, but using the new
API each element is creating its own instance.