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/node/effect/opacity/opacityeffect.cpp

@@ -0,0 +1,92 @@
+/*
+ * Oak Video Editor - Non-Linear Video Editor
+ * Copyright (C) 2025 Olive CE Team
+ *
+ * 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 "opacityeffect.h"
+
+#include "node/math/math/math.h"
+#include "node/sliderdisplaytype.h"
+
+namespace olive
+{
+
+#define super Node
+
+const QString OpacityEffect::k_texture_input = QStringLiteral("tex_in");
+const QString OpacityEffect::k_value_input = QStringLiteral("opacity_in");
+
+OpacityEffect::OpacityEffect()
+{
+	MathNode *math = new MathNode();
+	math->setParent(this);
+
+	math->set_operation(MathNode::k_op_multiply);
+
+	set_node_position_in_context(math, QPointF(0, 0));
+
+	add_input(k_texture_input, NodeValue::k_texture,
+			 InputFlags(k_input_flag_not_keyframable));
+
+	add_input(k_value_input, NodeValue::k_float, 1.0);
+	set_input_property(k_value_input, QStringLiteral("view"),
+					 slider::k_percentage);
+	set_input_property(k_value_input, QStringLiteral("min"), 0.0);
+	set_input_property(k_value_input, QStringLiteral("max"), 1.0);
+
+	set_flag(k_video_effect);
+	set_effect_input(k_texture_input);
+}
+
+void OpacityEffect::retranslate()
+{
+	super::retranslate();
+
+	set_input_name(k_texture_input, tr("Texture"));
+	set_input_name(k_value_input, tr("Opacity"));
+}
+
+ShaderCode OpacityEffect::get_shader_code(const ShaderRequest &request) const
+{
+	if (request.id == QStringLiteral("rgbmult")) {
+		return ShaderCode(
+			FileFunctions::read_file_as_string(":/shaders/opacity_rgb.frag"));
+	} else {
+		return ShaderCode(
+			FileFunctions::read_file_as_string(":/shaders/opacity.frag"));
+	}
+}
+
+void OpacityEffect::value(const NodeValueRow &value, const NodeGlobals &globals,
+						  NodeValueTable *table) const
+{
+	// If there's no texture, no need to run an operation
+	if (TexturePtr tex = value[k_texture_input].to_texture()) {
+		if (TexturePtr opacity_tex = value[k_value_input].to_texture()) {
+			ShaderJob job(value);
+			job.set_shader_id(QStringLiteral("rgbmult"));
+			table->push(NodeValue::k_texture, tex->to_job(job), this);
+		} else if (!qFuzzyCompare(value[k_value_input].to_double(), 1.0)) {
+			table->push(NodeValue::k_texture, tex->to_job(ShaderJob(value)),
+						this);
+		} else {
+			// 1.0 float is a no-op, so just push the texture
+			table->push(value[k_texture_input]);
+		}
+	}
+}
+
+}