build: split the engine into liboakengine.so; worker drops the UI entirely

Physical split: app/{audio,cli,codec,common,config,node,pluginSupport,
render,task,timeline,undo,tool,shaders} plus coreengine, version and
ui/icons+colorcoding move to a new top-level engine/ tree, built as
liboakengine.so (shared). The render backends (oakgl/oakvulkan) move
with it and link the engine library instead of embedding a static
render-core subset (libolive-rendercore is gone).

- oak-render-worker now links liboakengine instead of the whole
  libolive-editor object set: 336MB -> 2.9MB, no Qt Widgets UI
- the editor links liboakengine for the engine and keeps only UI
  objects in libolive-editor
- install/packaging: GNUInstallDirs libdir on Linux, bundle copy on
  macOS, oakengine.dll staged for NSIS, AppImage validation entry
- fix backend lookup for the new layout: DynamicRenderer searched
  ../app but backends now live in engine/; a stale pre-split liboakgl
  in the build tree got dlopened instead, re-initialized and later
  destroyed the interposed engine statics (full-suite segfault at
  DialogSequenceParameterTab, found via gdb watchpoint)

engine/codec/oiio/oiioencoder.cpp

@@ -0,0 +1,84 @@
+/***
+
+  Olive - Non-Linear Video Editor
+  Copyright (C) 2022 Olive Team
+  Modifications Copyright (C) 2025 mikesolar
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#include "oiioencoder.h"
+
+#include "common/oiioutils.h"
+
+namespace olive
+{
+
+OIIOEncoder::OIIOEncoder(const EncodingParams &params)
+	: Encoder(params)
+{
+}
+
+bool OIIOEncoder::open()
+{
+	return true;
+}
+
+bool OIIOEncoder::write_frame(FramePtr frame, Rational time)
+{
+	std::string filename = get_filename_for_frame(time).toStdString();
+
+	auto output = OIIO::ImageOutput::create(filename);
+	if (!output) {
+		return false;
+	}
+
+	OIIO::TypeDesc type = OIIOUtils::get_oiio_base_type_from_format(frame->format());
+	OIIO::ImageSpec spec(frame->width(), frame->height(),
+						 frame->channel_count(), type);
+
+	if (!output->open(filename, spec)) {
+		return false;
+	}
+
+	if (!output->write_image(type, frame->data(), OIIO::AutoStride,
+							 frame->linesize_bytes())) {
+		return false;
+	}
+
+	if (!output->close()) {
+		return false;
+	}
+
+	return true;
+}
+
+bool OIIOEncoder::write_audio(const SampleBuffer &audio)
+{
+	// Do nothing
+	return false;
+}
+
+bool OIIOEncoder::write_subtitle(const SubtitleBlock *sub_block)
+{
+	return false;
+}
+
+void OIIOEncoder::close()
+{
+	// Do nothing
+}
+
+}