fix(renderer): macOS TBDR cross-context sync and OFX instance thread-safety

This commit fixes crashes (SIGSEGV in CImg blur_bilateral) and black-frame
corruption artifacts during playback and scrubbing on macOS Apple Silicon.

Root cause analysis:
1. macOS uses Tile-Based Deferred Rendering (TBDR). glFlush() does not
   guarantee tile memory writeback, causing glReadPixels to read incomplete
   tiles (black/corrupted frames) and cache them to disk.
2. Olive uses multiple shared OpenGL contexts (RenderProcessor contexts vs.
   thread-local PluginRenderer context). glFinish() only waits for the
   current context, not the shared context that produced the texture. CPU
   readback in PluginRenderer could read partially-rendered tiles.
3. OlivePluginInstance and OliveClipInstance are not thread-safe. Concurrent
   RenderProcessors could corrupt internal QMap/images_ and params_ via
   setInputTexture/renderAction races.

Fixes:
- OpenGLRenderer::Flush() on macOS now uses glFinish() unconditionally.
- OpenGLRenderer::DownloadFromTexture() and OpenGLRenderer::Blit() insert
  glFinish() before readback/detach to ensure tile writeback completes.
- PluginRenderer::RenderPlugin() now acquires a per-instance mutex to
  serialize concurrent OFX render calls.
- Before CPU readback in PluginRenderer, flush the renderer that originally
  produced each input texture, ensuring cross-context synchronization.
- RenderProcessor::ProcessVideoFootage() flushes after BlitColorManaged.
- Add black-frame detection in ProcessVideoCacheJob() to auto-purge TBDR-
   corrupted cache files.
- Add diagnostic qDebug() logging in viewer, decoder, renderer, and plugin
  paths to aid future debugging.

app/render/opengl/openglrenderer.cpp

@@ -370,6 +370,9 @@ void OpenGLRenderer::DownloadFromTexture(const QVariant &id,
 
 	functions_->glPixelStorei(GL_PACK_ROW_LENGTH, linesize);
 
+	// Ensure all rendering is complete before reading back on TBDR architectures (macOS)
+	functions_->glFinish();
+
 	{
 		PRINT_GL_ERRORS;
 		functions_->glReadPixels(0, 0, p.effective_width(),
@@ -392,7 +395,14 @@ void OpenGLRenderer::Flush()
 	if (OLIVE_CONFIG("UseGLFinish").toBool()) {
 		functions_->glFinish();
 	} else {
+#if defined(Q_OS_MAC)
+		// macOS uses Tile-Based Deferred Rendering (TBDR). glFlush() does not
+		// guarantee that tile memory has been written back to texture memory.
+		// Using glFinish() prevents partial tile corruption ("black ink" artifacts).
+		functions_->glFinish();
+#else
 		functions_->glFlush();
+#endif
 	}
 }