iOS renderer using OpenGL ES 2

[ming]

This ticket creates OpenGL 2 framework to be used by other platforms and provides implementation for iOS with OpenGL ES version 2.

[ming]

In 4812:

Re #1757: Initial implementation of iOS OpenGL ES renderer. To use it, application needs to add:
#define PJMEDIA_VIDEO_DEV_HAS_OPENGL 1
#define PJMEDIA_VIDEO_DEV_HAS_OPENGL_ES 1
#define PJMEDIA_VIDEO_DEV_HAS_IOS_OPENGL 1

[ming]

In 4813:

Re #1757: Add example to display video in ipjsua (require ffmpeg libraries)

[ming]

In 4821:

Closed #1757: iOS OpenGL renderer

• Optimize rendering speed and CPU usage by avoiding buffer copy from frame's buffer
• Add various video dev capabilities

[ming]

In 4835:

Re #1757: Avoid calling OpenGL ES functions when in the background (which may cause app to be killed by iOS)

[ming]

In r4848:
Re #1757: Modify display video function in r4813 so it can display video preview as well

[bennylp]

In 4881:

Re #1757, re #1758: fix configure script. Now OpenGL renderer is enabled by default by the configure script. And added missing dependency to -lstdc++ for the openh274 library

[nanang]

In 4907:

Re #1757: Add iOS video renderer OpenGL macro settings to config.h.

[nanang]

In 4908:

Re #1757: Set OpenGL as default renderer on iOS (initialized before UIView renderer).