avmediaogl: use a create function for player

Change-Id: I4c2cc6e99f84a5e4fe78b179891a03e50485056c
Patching libgltf: character buffer is used as cstring

Change-Id: If0213fc5406ec9cbdc72be36ffc693fcbbf8ffcc
Extract code of BitmapEx -> RGBA buffer conversion to OpenGLHelper

Needed by gltf rendering.

Change-Id: I1aa974f3c515c5fb19a07b54ff655331138553cb