The API user is supposed to call this when the update callback was invoked
(like all mpv_render_* functions, this has to happen on the render thread,
and _not_ from the update callback itself).
This is optional if MPV_RENDER_PARAM_ADVANCED_CONTROL was not set (default).
Otherwise, it's a hard requirement that this is called after each update
callback. If multiple update callback happened, and the function could not
be called sooner, it's OK to call it once after the last callback.
If an update callback happens during or after this function, the function
must be called again at the soonest possible time.
If MPV_RENDER_PARAM_ADVANCED_CONTROL was set, this will do additional work
such as allocating textures for the video decoder.
@return a bitset of mpv_render_update_flag values (i.e. multiple flags are
combined with bitwise or). Typically, this will tell the API user
what should happen next. E.g. if the MPV_RENDER_UPDATE_FRAME flag is
set, mpv_render_context_render() should be called. If flags unknown
to the API user are set, or if the return value is 0, nothing needs
to be done.
The API user is supposed to call this when the update callback was invoked (like all mpv_render_* functions, this has to happen on the render thread, and _not_ from the update callback itself).
This is optional if MPV_RENDER_PARAM_ADVANCED_CONTROL was not set (default). Otherwise, it's a hard requirement that this is called after each update callback. If multiple update callback happened, and the function could not be called sooner, it's OK to call it once after the last callback.
If an update callback happens during or after this function, the function must be called again at the soonest possible time.
If MPV_RENDER_PARAM_ADVANCED_CONTROL was set, this will do additional work such as allocating textures for the video decoder.
@return a bitset of mpv_render_update_flag values (i.e. multiple flags are combined with bitwise or). Typically, this will tell the API user what should happen next. E.g. if the MPV_RENDER_UPDATE_FRAME flag is set, mpv_render_context_render() should be called. If flags unknown to the API user are set, or if the return value is 0, nothing needs to be done.