Add a new prepareToRender() callback for renders to perform their initial window clears

This consolidates all the clearing that was strewn across the codebase.

app/streaming/video/ffmpeg-renderers/drm.cpp

@@ -47,6 +47,7 @@ extern "C" {
 #include <sys/mman.h>
 
 #include "streaming/streamutils.h"
+#include "streaming/session.h"
 
 #include <Limelight.h>
 
@@ -178,6 +179,43 @@ bool DrmRenderer::prepareDecoderContext(AVCodecContext* context, AVDictionary**
     return true;
 }
 
+void DrmRenderer::prepareToRender()
+{
+    // 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. We have to do this before we enumerate
+    // CRTC modes below.
+    SDL_Renderer* renderer = SDL_CreateRenderer(params->window, -1, SDL_RENDERER_SOFTWARE);
+    if (renderer != nullptr) {
+        // 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().
+        Session* session = Session::get();
+        if (session != nullptr) {
+            // If we get here during a session, we need to synchronize with the event loop
+            // to ensure we don't drop any important events.
+            session->flushWindowEvents();
+        }
+        else {
+            // If we get here prior to the start of a session, just pump and flush ourselves.
+            SDL_PumpEvents();
+            SDL_FlushEvent(SDL_WINDOWEVENT);
+        }
+
+        SDL_SetRenderDrawColor(renderer, 0, 0, 0, SDL_ALPHA_OPAQUE);
+        SDL_RenderClear(renderer);
+        SDL_RenderPresent(renderer);
+        SDL_DestroyRenderer(renderer);
+    }
+    else {
+        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
+                     "SDL_CreateRenderer() failed: %s",
+                     SDL_GetError());
+    }
+}
+
 bool DrmRenderer::getPropertyByName(drmModeObjectPropertiesPtr props, const char* name, uint64_t *value) {
     for (uint32_t j = 0; j < props->count_props; j++) {
         drmModePropertyPtr prop = drmModeGetProperty(m_DrmFd, props->props[j]);