gl/cocoa: macos 10.15 access to contentView inside main UI thread bugfix

gstreamer/gst-plugins-base#686
gstreamer/gst-plugins-base#685

macos 10.15 catalina requires access operations of
[NSWindow contentView] must be inside of the main UI thread
gst_gl_window_cocoa_draw/gst_gl_window_cocoa_queue_resize
26 jobs for macos_10_15_UI_API_called_on_a_background_thread in 44 minutes and 22 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #853890
gst indent

00:00:30

passed #853889
manifest

00:00:41

 
  Build
passed #853900
build cerbero cross-android universal

00:18:15

manual #853904
gst-ios-12.2 allowed to fail manual
build cerbero cross-ios universal
passed #853901
build cerbero cross win32

00:10:19

passed #853902
build cerbero cross win64

00:07:00

passed #853899
build cerbero fedora x86_64

00:06:52

passed #853903
gst-macos-10.14
build cerbero macos x86_64

00:07:00

passed #853895
build clang fedora x86_64

00:06:07

passed #853891
build fedora x86_64

00:04:03

passed #853898
docker windows 1607
build msys2

00:11:55

passed #853892
build nodebug fedora x86_64

00:03:01

passed #853893
build static fedora x86_64

00:13:26

passed #853894
build static nodebug fedora x86_64

00:04:12

passed #853896
docker windows 1607
build vs2017 amd64

00:10:10

passed #853897
docker windows 1607
build vs2017 x86

00:09:37

 
  Test
passed #853905
check fedora

00:04:03

passed #853906
integration testsuites fedora

00:39:32

passed #853910
valgrind bad

00:09:31

passed #853907
valgrind base

00:07:39

passed #853911
valgrind ges

00:06:34

passed #853908
valgrind good

00:11:23

passed #853909
valgrind ugly

00:03:10

 
  Integrate
passed #853912
cross-android universal examples

00:08:03

manual #853914
gst-ios-12.2 allowed to fail manual
cross-ios universal examples
passed #853913
documentation

00:03:42