Crash in iOS OpenGL renderer when in background and assertion during destruction

[ming]

The current iOS OpenGL renderer has two problems:

• Crash when created in the background.
  When app accepts video call when in bg, without going to foreground, creating OpenGL context will crash.

• Assertion caused by race condition between rendering and destruction

  Assertion failed: (vid_tee && vid_tee->info.signature==( 'E'<<24 | 'T'<<16 | 'V'<<8 | 'P' )), function pjmedia_vid_tee_remove_dst_port, file ../src/pjmedia/vid_tee.c, line 277.
  
  #4	0x0012e18a in pjmedia_vid_tee_remove_dst_port at pjproject-2.0-iphone/pjmedia/build/../src/pjmedia/vid_tee.c:276
  #5	0x001e60a0 in pjsua_vid_stop_stream at pjproject-2.0-iphone/pjsip/build/../src/pjsua-lib/pjsua_vid.c:1010
  #6	0x001dd894 in stop_media_stream at pjproject-2.0-iphone/pjsip/build/../src/pjsua-lib/pjsua_media.c:2238
  #7	0x001dcc10 in stop_media_session [inlined] at pjproject-2.0-iphone/pjsip/build/../src/pjsua-lib/pjsua_media.c:2285
  

[ming]

In 5047:

Fixed #1835: Fixed crash in iOS OpenGL renderer when in background and assertion during destruction

[ming]

In 5052:

Re #1835: More robust handling of format change, to prevent data race if clock is still running.