gltestsrc: fix shaders ref counting

gltestsrc: fix shaders ref counting

The gltestsrc element uses two shaders: color_shader and snow_shader.
Those are alternatively assigned to the SrcShader->shader pointer and
their reference was transferred to it. Only the SrcShader->shader was
unreffed (in _src_shader_deinit()) so only one shader was properly
freed, the other one was leaked.

Fixed this by giving an extra ref to SrcShader->shader and unreffing the
2 shaders in _src_smpte_free().

https://bugzilla.gnome.org/show_bug.cgi?id=766661