Move forced KMSDRM modesetting operation into Session init code

Performing this each time DrmRenderer is initialized leads to long
delays when choosing a decoder on embedded platforms, particularly
those like TH1520/JH7110 which lack accelerated GL drivers.

app/streaming/session.cpp

@@ -1802,6 +1802,31 @@ void Session::execInternal()
             FillRect(info.info.win.hdc, &clientRect, blackBrush);
             DeleteObject(blackBrush);
         }
+#else
+        if (strcmp(SDL_GetCurrentVideoDriver(), "KMSDRM") == 0) {
+            // Create a dummy renderer to force SDL to complete the modesetting
+            // operation that the KMSDRM backend keeps pending until the next
+            // time we swap buffers.
+            SDL_Renderer* renderer = SDL_CreateRenderer(m_Window, -1, SDL_RENDERER_SOFTWARE);
+            if (renderer != nullptr) {
+                SDL_SetRenderDrawColor(renderer, 0, 0, 0, SDL_ALPHA_OPAQUE);
+                SDL_RenderClear(renderer);
+                SDL_RenderPresent(renderer);
+                SDL_DestroyRenderer(renderer);
+
+                // SDL_CreateRenderer() can end up having to recreate our window (SDL_RecreateWindow())
+                // to ensure it's compatible with the renderer's OpenGL context. If that happens, we
+                // can get spurious SDL_WINDOWEVENT events that will cause us to (again) recreate our
+                // renderer. This can lead to an infinite to renderer recreation, so discard all
+                // SDL_WINDOWEVENT events after SDL_CreateRenderer().
+                flushWindowEvents();
+            }
+            else {
+                SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
+                             "SDL_CreateRenderer() for KMSDRM modesetting failed: %s",
+                             SDL_GetError());
+            }
+        }
 #endif
     }