libobs: Allow transitions to give placeholder

The fade transition could benefit by providing NULL to differentiate a
real source texture from the transparent placeholder. This would give it
a chance to fade correctly for source transitions.

docs/sphinx/reference-sources.rst

@@ -1576,6 +1576,7 @@ Functions used by transitions
 ---------------------
 
 .. function:: void obs_transition_video_render(obs_source_t *transition, obs_transition_video_render_callback_t callback)
+              void obs_transition_video_render2(obs_source_t *transition, obs_transition_video_render_callback_t callback, gs_texture_t *placeholder_texture)
 
    Helper function used for rendering transitions.  This function will
    render two distinct textures for source A and source B of the
@@ -1587,6 +1588,10 @@ Functions used by transitions
    (0.0f..1.0f), *cx* and *cy* are the current dimensions of the
    transition, and *data* is the implementation's private data.
 
+   The *placeholder_texture* parameter allows a callback to receive
+   a replacement that isn't the default transparent texture, including
+   NULL if the caller desires.
+
    Relevant data types used with this function:
 
 .. code:: cpp