Alpha in GStreamer is unpremultiplied
#define DEFAULT_PAD_BLEND_EQUATION_RGB GST_GL_VIDEO_MIXER_BLEND_EQUATION_ADD
#define DEFAULT_PAD_BLEND_EQUATION_ALPHA GST_GL_VIDEO_MIXER_BLEND_EQUATION_ADD
#define DEFAULT_PAD_BLEND_FUNCTION_SRC_RGB GST_GL_VIDEO_MIXER_BLEND_FUNCTION_SRC_ALPHA
-#define DEFAULT_PAD_BLEND_FUNCTION_SRC_ALPHA GST_GL_VIDEO_MIXER_BLEND_FUNCTION_SRC_ALPHA
+#define DEFAULT_PAD_BLEND_FUNCTION_SRC_ALPHA GST_GL_VIDEO_MIXER_BLEND_FUNCTION_ONE
#define DEFAULT_PAD_BLEND_FUNCTION_DST_RGB GST_GL_VIDEO_MIXER_BLEND_FUNCTION_ONE_MINUS_SRC_ALPHA
#define DEFAULT_PAD_BLEND_FUNCTION_DST_ALPHA GST_GL_VIDEO_MIXER_BLEND_FUNCTION_ONE_MINUS_SRC_ALPHA