refactor(plugin): de-Qt oakplugin and wrap it in a pure C ABI

- de-Qt src/plugin/src (olivehost/oliveplugininstance/oliveclip/
  paraminstance/image/pluginprogressreporter); QMessageBox/
  QApplication replaced by facade callbacks
- new C ABI in include/plugin/{error,host,instance}.h with
  refcounted OakPluginInstance value handle
- paraminstance bridges via oaknode C ABI (OakNodeNode value handle,
  oaknode_node_identity registry); undo via oakundo C ABI
- oliveclip textures are OakRenderTexture value handles; oakrender
  gains texture/copier/ticket C API additions
- oaknode gains get_input_at_time/set_input_at_time_undoable/
  node_identity/sequence_from_node/sequence_set_default_parameters/
  find_input_footage
- move avframeptr.h to src/common/src (shared by codec and render)
- node transition pluginSupport stubs bridge the real oakplugin
  headers; all oaknode-building standalone trees add src/plugin and
  link oakplugin into their test binaries
- plugin tests self-sufficient (ipc shim, OfxHost force_load,
  PRE_TEST discovery, OCIO env); timeline standalone gains
  render/c_api
- restore ffmpegdecoder.cpp in src/codec/src/ffmpeg/CMakeLists.txt
  (dropped in 3d004c081, caused jump-to-0 in decoder/encoder tests)

All six standalone trees green: codec 22, audio 40, task 110,
render 49, timeline 121, plugin 100.

include/render/renderer.h

@@ -23,6 +23,10 @@
 
 #include <stdint.h>
 
+#ifdef __cplusplus
+#include <memory>
+#endif
+
 #include "error.h"
 
 #ifdef __cplusplus
@@ -229,9 +233,42 @@ int oakrender_display_texture_download(OakRenderTexture texture, void *pixels,
 int oakrender_display_texture_get_params(OakRenderTexture texture,
 										 oakrender_video_params *out);
 
+/** @brief Frame width/height in pixels (0 on empty). */
+int oakrender_codec_frame_width(OakCodecFrame frame);
+int oakrender_codec_frame_height(OakCodecFrame frame);
+
+/** @brief ffmpeg_bridge pixel format when the frame wraps a texture's
+ *        CPU copy (an AVFramePtr); -1 otherwise. */
+int oakrender_codec_frame_fb_format(OakCodecFrame frame);
+
 /** @brief Native texture id (0 on empty or a dummy/id-less texture). */
 int oakrender_display_texture_id(OakRenderTexture texture);
 
+/** @brief 1 when the texture is a placeholder dummy (Texture::is_dummy()). */
+int oakrender_display_texture_is_dummy(OakRenderTexture texture);
+
+/**
+ * @brief The CPU frame stored in the texture, if any (Texture::frame()).
+ *        *out receives a retained frame handle (empty when none).
+ */
+int oakrender_display_texture_get_frame(OakRenderTexture texture,
+										OakCodecFrame *out);
+
+#ifdef __cplusplus
+} /* extern "C" */
+
+namespace olive { class Texture; using TexturePtr = std::shared_ptr<Texture>; }
+
+/**
+ * @brief Wrap a native TexturePtr in a retained handle (C++ only; used
+ *        by oakrender internals when handing textures across the C ABI).
+ */
+OakRenderTexture oakrender_display_texture_wrap_native(
+	const olive::TexturePtr &texture);
+
+extern "C" {
+#endif
+
 /* ---- Frame handle -------------------------------------------------------- */
 
 /** @brief Create an empty CPU frame. Returns a handle with count 1. */