glviewconvert: fix output when a transformation matrix is used

If we have a transformation matrix, we have no idea where in the output
the video is going to endup.  It might also be different and not cover
the entire output.

We need to clear the output to remove any previous data in the backing
texture.

Found from
https://stackoverflow.com/questions/51707229/python-gstreamer-for-dynamic-control-of-element-properties

diff --git a/gst-libs/gst/gl/gstglviewconvert.c b/gst-libs/gst/gl/gstglviewconvert.c
index d032c21..ae7bb1f 100644 (file)
--- a/gst-libs/gst/gl/gstglviewconvert.c
+++ b/gst-libs/gst/gl/gstglviewconvert.c
@@ -1949,6 +1949,9 @@ _do_view_convert_draw (GstGLContext * context, GstGLViewConvert * viewconvert)
   gl->ActiveTexture (GL_TEXTURE0);
   gl->BindTexture (from_gl_target, priv->in_tex[0]->tex_id);
 
+  gl->ClearColor (0.0, 0.0, 0.0, 1.0);
+  gl->Clear (GL_COLOR_BUFFER_BIT);
+
   gl->DrawElements (GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, NULL);
 
   if (gl->BindVertexArray)