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/codec/ffmpeg/ffmpegdecoder.cpp

@@ -349,6 +349,21 @@ TexturePtr FFmpegDecoder::RetrieveVideoInternal(const RetrieveVideoParams &p)
 			return nullptr;
 		}
 
+		// Diagnostic: check if decoded frame is all black
+		bool all_black = true;
+		if (f->data[0]) {
+			int check_rows = std::min(f->height, 8);
+			int check_bytes = check_rows * f->linesize[0];
+			for (int i = 0; i < check_bytes; ++i) {
+				if (f->data[0][i] != 0) {
+					all_black = false;
+					break;
+				}
+			}
+		}
+		qDebug() << "[DECODER] RetrieveVideoInternal time=" << p.time.toDouble()
+				 << "format=" << static_cast<int>(f->format) << "black=" << all_black;
+
 		// Finally, perform any GPU processing required
 		TexturePtr texture = ProcessFrameIntoTexture(f, p, original);