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.

src/render/c_api/renderer.cpp

@@ -363,6 +363,9 @@ void *oakrender_codec_frame_data(OakCodecFrame frame)
 	if (!f || !f->ptr) {
 		return nullptr;
 	}
+	if (f->avptr) {
+		return f->avptr->data(0);
+	}
 	return f->ptr->data();
 }
 
@@ -380,6 +383,9 @@ int oakrender_codec_frame_linesize_bytes(OakCodecFrame frame)
 {
 	OakCodecFrameImpl *f =
 		oakrender_c_api::to_native<OakCodecFrameImpl>(frame);
+	if (f && f->avptr) {
+		return f->avptr->linesize(0);
+	}
 	if (!f || !f->ptr) {
 		return 0;
 	}
@@ -508,3 +514,84 @@ int oakrender_current_backend(char *buf, int n)
 	}
 	return write_string(g_requested_backend, buf, n);
 }
+
+int oakrender_display_texture_is_dummy(OakRenderTexture texture)
+{
+	if (!texture.ctx) {
+		return 0;
+	}
+	return oakrender_c_api::to_native<OakRenderTextureImpl>(texture)
+				   ->ptr->is_dummy()
+			   ? 1
+			   : 0;
+}
+
+int oakrender_display_texture_get_frame(OakRenderTexture texture,
+										OakCodecFrame *out)
+{
+	if (!texture.ctx || !out) {
+		return OAKRENDER_E_INVALID;
+	}
+	*out = OakCodecFrame{};
+
+	olive::AVFramePtr frame =
+		oakrender_c_api::to_native<OakRenderTextureImpl>(texture)->ptr->frame();
+	if (!frame) {
+		return OAKRENDER_E_NOT_FOUND;
+	}
+	// The texture's CPU copy is an AVFramePtr (ffmpeg_bridge RAII); wrap
+	// the shared_ptr so the frame stays alive with the handle.
+	auto *fimpl = new OakCodecFrameImpl;
+	fimpl->avptr = frame;
+	*out = oakrender_c_api::make_handle<OakCodecFrame>(
+		fimpl, true, &oakrender_c_api::delete_as<OakCodecFrameImpl>);
+	return out->ctx ? OAKRENDER_OK : OAKRENDER_E_NOMEM;
+}
+
+OakRenderTexture oakrender_display_texture_wrap_native(
+	const olive::TexturePtr &texture)
+{
+	if (!texture) {
+		return OakRenderTexture{};
+	}
+	auto *impl = new OakRenderTextureImpl;
+	impl->ptr = texture;
+	return oakrender_c_api::make_handle<OakRenderTexture>(
+		impl, true, &oakrender_c_api::delete_as<OakRenderTextureImpl>);
+}
+
+int oakrender_codec_frame_width(OakCodecFrame frame)
+{
+	OakCodecFrameImpl *f =
+		oakrender_c_api::to_native<OakCodecFrameImpl>(frame);
+	if (!f) {
+		return 0;
+	}
+	if (f->avptr) {
+		return f->avptr->width();
+	}
+	return f->ptr ? f->ptr->width() : 0;
+}
+
+int oakrender_codec_frame_height(OakCodecFrame frame)
+{
+	OakCodecFrameImpl *f =
+		oakrender_c_api::to_native<OakCodecFrameImpl>(frame);
+	if (!f) {
+		return 0;
+	}
+	if (f->avptr) {
+		return f->avptr->height();
+	}
+	return f->ptr ? f->ptr->height() : 0;
+}
+
+int oakrender_codec_frame_fb_format(OakCodecFrame frame)
+{
+	OakCodecFrameImpl *f =
+		oakrender_c_api::to_native<OakCodecFrameImpl>(frame);
+	if (!f || !f->avptr) {
+		return -1;
+	}
+	return f->avptr->format();
+}