Add OpenGL as source pad capability

[nuess0r]

Hi

Thank you very much for the gstreamer plugin. It makes my life to play around with libprojectm a lot easier and opens up lots of possibilities.

For some use cases it's convenient, that gst-projectm spits out a video stream but I was a little bit surprised to see that there is no OpenGL available as source capability. In gstreamer pipelines there are lots of OpenGL modules available that would avoid copying the gst-projectm output to the CPU and later back to the GPU.

Source pad capabilities of gst-projectm:

  SRC template: 'src'
    Availability: Always
    Capabilities:
      video/x-raw
                 format: { (string)RGBA }
              framerate: [ 0/1, 2147483647/1 ]
                  width: [ 1, 2147483647 ]
                 height: [ 1, 2147483647 ]

Here an example pipeline that works with the current gst-projectm:

gst-launch-1.0 audiotestsrc ! queue ! audioconvert ! projectm ! video/x-raw,width=640,height=480,framerate=30/1 ! videoconvert ! glupload ! glimagesink

Here how it should work when the GL texture can be directly used:

gst-launch-1.0 audiotestsrc ! queue ! audioconvert ! projectm ! video/x-raw,width=640,height=480,framerate=30/1 ! glimagesink

I don't know if it is feasible or wished to combine it with the fbo rendering patch #12