lavfi: add a new filtergraph parsing API

Callers currently have two ways of adding filters to a graph - they can
either
- create, initialize, and link them manually
- use one of the avfilter_graph_parse*() functions, which take a
  (typically end-user-written) string, split it into individual filter
  definitions+options, then create filters, apply options, initialize
  filters, and finally link them - all based on information from this
  string.

A major problem with the second approach is that it performs many
actions as a single atomic unit, leaving the caller no space to
intervene in between. Such intervention would be useful e.g. to
- modify filter options;
- supply hardware device contexts;
both of which typically must be done before the filter is initialized.

Callers who need such intervention are then forced to invent their own
filtergraph parsing, which is clearly suboptimal.

This commit aims to address this problem by adding a new modular
filtergraph parsing API. It adds a new  avfilter_graph_segment_parse()
function to parse a string filtergraph description into an intermediate
tree-like representation (AVFilterGraphSegment and its children).

This intermediate form may then be applied step by step using further
new avfilter_graph_segment*() functions, with user intervention possible
between each step.

1 files changed, 15 insertions, 0 deletions

diff --git a/doc/APIchanges b/doc/APIchanges
index 810b8c0746..2c4723c669 100644
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -2,6 +2,21 @@ The last version increases of all libraries were on 2023-02-09
 
 API changes, most recent first:
 
+2023-02-12 - xxxxxxxxxx - lavfi 9.1.100 - avfilter.h
+  Add filtergraph segment parsing API.
+  New structs:
+    - AVFilterGraphSegment
+    - AVFilterChain
+    - AVFilterParams
+    - AVFilterPadParams
+  New functions:
+    - avfilter_graph_segment_parse()
+    - avfilter_graph_segment_create_filters()
+    - avfilter_graph_segment_apply_opts()
+    - avfilter_graph_segment_init()
+    - avfilter_graph_segment_link()
+    - avfilter_graph_segment_apply()
+
 2023-02-xx - xxxxxxxxxx - lavu 58.0.100 - csp.h
   Add av_csp_approximate_trc_gamma() and av_csp_trc_func_from_id().
   Add av_csp_trc_function.