flutter-pi: make context current for engine run

FlutterEngineRunInitialized resolves the GL procs in the same thread
when using impeller, so an EGL context needs to be current. Make the
setup context current for the duration of FlutterEngineRunInitialized.

Also, this means that we can't use the setup context in the
`on_make_current` callback anymore, since that's called on the
rasterizer thread, possibly while `FlutterEngineRunInitialized` is
still running. Just use the rendering context there. (No real reason
to use the setup context anyway)

src/flutter-pi.c

@@ -316,7 +316,13 @@ static bool on_make_current(void *userdata) {
     // }
 
     // The actual surface will be made current later.
-    ok = gl_renderer_make_flutter_setup_context_current(flutterpi->gl_renderer);
+    // 
+    // We use the rendering context here instead of the setup context
+    // because this is called on the rasterizer thread, and the setup
+    // context might still be current on the main (platform) thread.
+    // (Because for impeller, we need to have a context current for
+    // FlutterEngineRunInitialized)
+    ok = gl_renderer_make_flutter_rendering_context_current(flutterpi->gl_renderer, EGL_NO_SURFACE);
     if (ok != 0) {
         return false;
     }
@@ -1420,13 +1426,43 @@ static int flutterpi_run(struct flutterpi *flutterpi) {
 
     flutterpi->flutter.engine = engine;
 
+    // When running with `--enable-impeller`, the GL context must be made current
+    // before calling `FlutterEngineRunInitialized`, because impeller
+    // resolves the GL procs inside RunInitialized.
+    if (flutterpi->gl_renderer != NULL) {
+#ifdef HAVE_EGL_GLES2
+        ok = gl_renderer_make_flutter_setup_context_current(flutterpi->gl_renderer);
+        if (ok != 0) {
+            goto fail_deinitialize_engine;
+        }
+#else
+        UNREACHABLE();
+#endif
+    }
+
     engine_result = procs->RunInitialized(engine);
     if (engine_result != kSuccess) {
         LOG_ERROR("Could not run the flutter engine. FlutterEngineRunInitialized: %s\n", FLUTTER_RESULT_TO_STRING(engine_result));
         ok = EIO;
         goto fail_deinitialize_engine;
     }
 
+    // We need to clear the context because flutter then tries to make the
+    // rendering context current on a different (rasterizer) thread.
+    /// TODO: Is this a race condition (if we're not fast enough the clear the
+    ///  current context before the rasterizer thread tries to make the context
+    ///  current?)
+    if (flutterpi->gl_renderer != NULL) {
+#ifdef HAVE_EGL_GLES2
+        ok = gl_renderer_clear_current(flutterpi->gl_renderer);
+        if (ok != 0) {
+            goto fail_deinitialize_engine;
+        }
+#else
+        UNREACHABLE();
+#endif
+    }
+
     ok = locales_add_to_fl_engine(flutterpi->locales, engine, procs->UpdateLocales);
     if (ok != 0) {
         goto fail_shutdown_engine;