gl: font rendering element

Submitted by Julien Isorce @cap

Link to original bug (#723761)

Description

Similar to textoverlay element.

Use an external GL Font API to build a separate gltexture and attach it to the input buffer's GstVideoOverlayCompositionMeta.

Then a downstream element could call gst_video_overlay_composition_blend to show up the subtitles.

(from gstvideooverlaycomposition API
http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gst-plugins-base-libs-gstvideooverlaycomposition.html)

Could inherit from GstBaseTextOverlay.

gst-launch-1.0 filesrc location=text.srt ! subparse ! text/x-raw ! gltextoverlay.text_sink ! glimagesink

Could be a separate gl element (gltextoverlay) or in GstGLFilter.

Could use FTGL https://sourceforge.net/projects/ftgl/ or any other GL font lib.