[Bug 759867] New: text rendering is not opengl accelerated properly

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=759867

            Bug ID: 759867
           Summary: text rendering is not opengl accelerated properly
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-bad
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: florent.thiery at ubicast.eu
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

My use case is to render text overlays (textoverlay/timeoverlay) with GLES on
Raspberrypi ; i apply a fragment shader and the timeoverlay is currently
directly rendered by the sink and does not get processed through the shader:

gst-launch-1.0 videotestsrc ! glupload ! timeoverlay ! glshader
location=oculus.frag ! glimagesink

(shader here:
https://github.com/fthiery/rpi-oculus-fpv/blob/master/rpi-oculus-fpv/oculus.frag)

However, for testing sake, the issue can be reproduced using any gl effect like
glfiltercube; the expected result is to see the timeoverlay on every cube face
instead of the top left sink corner:

gst-launch-1.0 videotestsrc ! glupload ! timeoverlay ! glcolorconvert !
glfiltercube ! glimagesink

The expected result can be obained by moving timeoverlay before glupload (but
in that case, text rendering is done in software, which i cannot afford and may
require unnecessary texture download/upload):

gst-launch-1.0 videotestsrc ! timeoverlay ! glupload ! glcolorconvert !
glfiltercube ! glimagesink

Also, why is the following failing to link (resulting in requiring a
glcolorconvert element) ?
gst-launch-1.0 videotestsrc ! glupload ! timeoverlay ! glfiltercube !
glimagesink
WARNING: erroneous pipeline: could not link timeoverlay0 to glfiltercube0

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