Memory leak in setting default video codec parameters (thanks Bo Shi for the feedback)

[nanang]

Calling pjmedia_vid_codec_mgr_set_default_param() multiple times using the same pool instance will cause the pool bloating. This is actually expected behaviour. It was assumed that application won't need to change the default codec param frequently. And if application needs to, it should manage the pool by itself, i.e: create new pool before and release the old one after calling the function, to avoid the pool bloating.

However, after reconsider it again, frequent changes on the default codec param can be common case for video applications, so we think it is better to let the codec framework to manage the pool.

[nanang]

(In [3956]) Fix #1453:

• Add and manage pool instance in default codec param in video codec framework.
• API change: pool param is removed from pjmedia_vid_codec_mgr_set_default_param().

[nanang]

(In [4008]) Re #1453: fixed memory leak in pjmedia_vid_codec_mgr_set_default_param() caused by unreleased old pool when the new codec param is not NULL (thanks Faba Aa for the report).