- update for shaders
- add alpha property
- image placement properties shamelessly borrowed from gdkpixbufoverlay
- image placement properties are GstController able
- use GstGLMemory for the overlay image data
- add support for gles2
This is too allow gst-launch debugging with multiple GL contexts as
well as avoiding segfaulting innocent gtk+ apps that have not called
XInitThreads.
https://bugzilla.gnome.org/show_bug.cgi?id=731525