glcompositionoverlay: Add class for managing GstVideoOverlayCompositionOverlay buffers

glcompositionoverlay: Add class for managing GstVideoOverlayCompositionOverlay buffers

Add a class to store and manage the OpenGL texture,
vertex buffer and GstVideoOverlayRectangle.
Transforms overlay coordinate space to vertex buffer space with aspect ratios in mind.

= Example Pipelines =

Simple pipeline

gst-launch-1.0 videotestsrc ! \
  textoverlay text="Hello World" font-desc="sans bold 30" ! \
  glimagesink

Display 3 static overlays at different positions

gst-launch-1.0 videotestsrc ! \
  textoverlay text="text1" valignment="top" font-desc="sans bold 30" ! \
  textoverlay text="text2" halignment="right" font-desc="sans bold 30" ! \
  textoverlay text="text3" halignment="left" font-desc="sans bold 30" ! \
  glimagesink

Display subtitle file over testsrc

gst-launch-1.0 videotestsrc ! \
  textoverlay name=foo filesrc location=foo.srt ! subparse ! queue ! foo. foo. ! \
  glimagesink

https://bugzilla.gnome.org/show_bug.cgi?id=745107