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/shaders/ripple.frag

@@ -0,0 +1,38 @@
+uniform float evolution_in;
+uniform float intensity_in;
+uniform float frequency_in;
+uniform vec2 position_in;
+uniform bool stretch_in;
+
+uniform vec2 resolution_in;
+uniform sampler2D tex_in;
+
+in vec2 ove_texcoord;
+out vec4 frag_color;
+
+void main(void) {
+  vec2 center = position_in/resolution_in;
+
+  vec2 adj_texcoord = ove_texcoord;
+
+  adj_texcoord -= 0.5;
+  if (!stretch_in) {
+    // Adjust by aspect ratio
+    float ar = (resolution_in.x/resolution_in.y);
+    if (resolution_in.x > resolution_in.y) {
+      adj_texcoord.y /= ar;
+      center.y /= ar;
+    } else {
+      adj_texcoord.x *= ar;
+      center.x *= ar;
+    }
+  }
+  adj_texcoord += 0.5;
+  center += 0.5;
+
+  adj_texcoord -= center;
+
+  float len = length(adj_texcoord);
+  vec2 uv = ove_texcoord + (adj_texcoord/len)*cos((frequency_in)*(len*12.0-evolution_in))*(intensity_in*0.0005);
+  frag_color = texture(tex_in, uv);
+}