Each plugin must provide its own unique key. Otherwise we will only ever
see one single plugin.

Right now running on i.MX6 is often broken because the imx6 videonode plugin is
not picked up since only the egl one is seen by the system. With the fix both plugins
provide their own unique key so both become visible.

Additionally, introduce a QT_VIDEONODE environment variable. This is useful to specify
which plugin to use. This is necessary in case multiple custom videonode plugins support
the same formats.

Change-Id: Iaa1988f8436dcb938cb9a95e2e0d68a4e92e113c
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>

[ChangeLog] Added a VideoNode plugin which allows direct rendering of
EGLImageKHR backed video frames.

Change-Id: I36fb6fd27680dbe9c71a446bbd54df95488725f8
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

Removed the overhead of having to create a shared OpenGL context in the GUI
thread and pre-render the frame into a FBO.

We now directly render the GL_TEXTURE_EXTERNAL_OES in the QtQuick render
thread, using an Android-specific QSGVideoNode.
We also use a callback from the render thread to create the texture from
there and not have to create a separate shared OpenGL context.

Change-Id: I6c8eb94b47d0a03329c912701b8af3fb5ebd1876
Reviewed-by: Christian Stromme <christian.stromme@digia.com>