Intialize gl client side API in MediaChildThread

[JulienIsorce]

Currently it is done in MediaPlayerGStreamer for the first video tag. Because initially we thought having on glthread per video tag. But this is actually overkill. It is better to have only one IPC channel with the GPU process.

So we can now move "::gles2::Initialize()" to MediaChildThread and properly call "::gles2::deinit()"