Fix: ensure switching blitter happens in the main thread

This because video-drivers might need to make changes to their
context, which for most video-drivers has to be done in the same
thread as the window was created; main thread in our case.

src/gfxinit.cpp

@@ -308,18 +308,11 @@ static bool SwitchNewGRFBlitter()
 		}
 		if (BlitterFactory::GetBlitterFactory(repl_blitter) == nullptr) continue;
 
-		DEBUG(misc, 1, "Switching blitter from '%s' to '%s'... ", cur_blitter, repl_blitter);
-		Blitter *new_blitter = BlitterFactory::SelectBlitter(repl_blitter);
-		if (new_blitter == nullptr) NOT_REACHED();
-		DEBUG(misc, 1, "Successfully switched to %s.", repl_blitter);
+		/* Inform the video driver we want to switch blitter as soon as possible. */
+		VideoDriver::GetInstance()->ChangeBlitter(repl_blitter);
 		break;
 	}
 
-	if (!VideoDriver::GetInstance()->AfterBlitterChange()) {
-		/* Failed to switch blitter, let's hope we can return to the old one. */
-		if (BlitterFactory::SelectBlitter(cur_blitter) == nullptr || !VideoDriver::GetInstance()->AfterBlitterChange()) usererror("Failed to reinitialize video driver. Specify a fixed blitter in the config");
-	}
-
 	return true;
 }
 

src/video/video_driver.cpp

@@ -10,8 +10,11 @@
 #include "../stdafx.h"
 #include "../core/random_func.hpp"
 #include "../network/network.h"
+#include "../blitter/factory.hpp"
 #include "../debug.h"
+#include "../fontcache.h"
 #include "../gfx_func.h"
+#include "../gfxinit.h"
 #include "../progress.h"
 #include "../thread.h"
 #include "../window_func.h"
@@ -74,6 +77,27 @@ void VideoDriver::StopGameThread()
 	this->game_thread.join();
 }
 
+void VideoDriver::RealChangeBlitter(const char *repl_blitter)
+{
+	const char *cur_blitter = BlitterFactory::GetCurrentBlitter()->GetName();
+
+	DEBUG(driver, 1, "Switching blitter from '%s' to '%s'... ", cur_blitter, repl_blitter);
+	Blitter *new_blitter = BlitterFactory::SelectBlitter(repl_blitter);
+	if (new_blitter == nullptr) NOT_REACHED();
+	DEBUG(driver, 1, "Successfully switched to %s.", repl_blitter);
+
+	if (!this->AfterBlitterChange()) {
+		/* Failed to switch blitter, let's hope we can return to the old one. */
+		if (BlitterFactory::SelectBlitter(cur_blitter) == nullptr || !this->AfterBlitterChange()) usererror("Failed to reinitialize video driver. Specify a fixed blitter in the config");
+	}
+
+	/* Clear caches that might have sprites for another blitter. */
+	this->ClearSystemSprites();
+	ClearFontCache();
+	GfxClearSpriteCache();
+	ReInitAllWindows();
+}
+
 void VideoDriver::Tick()
 {
 	if (!this->is_game_threaded && std::chrono::steady_clock::now() >= this->next_game_tick) {
@@ -115,6 +139,11 @@ void VideoDriver::Tick()
 
 			this->LockVideoBuffer();
 
+			if (this->change_blitter != nullptr) {
+				this->RealChangeBlitter(this->change_blitter);
+				this->change_blitter = nullptr;
+			}
+
 			while (this->PollEvent()) {}
 			::InputLoop();
 

src/video/video_driver.hpp

@@ -35,7 +35,7 @@ class VideoDriver : public Driver {
 	const uint DEFAULT_WINDOW_HEIGHT = 480u; ///< Default window height.
 
 public:
-	VideoDriver() : is_game_threaded(true) {}
+	VideoDriver() : is_game_threaded(true), change_blitter(nullptr) {}
 
 	/**
 	 * Mark a particular area dirty.
@@ -161,6 +161,15 @@ public:
 		return ZOOM_LVL_OUT_4X;
 	}
 
+	/**
+	 * Queue a request to change the blitter. This is not executed immediately,
+	 * but instead on the next draw-tick.
+	 */
+	void ChangeBlitter(const char *new_blitter)
+	{
+		this->change_blitter = new_blitter;
+	}
+
 	/**
 	 * Get the currently active instance of the video driver.
 	 */
@@ -303,6 +312,9 @@ protected:
 private:
 	void GameLoop();
 	void GameThread();
+	void RealChangeBlitter(const char *repl_blitter);
+
+	const char *change_blitter; ///< Request to change the blitter. nullptr if no pending request.
 };
 
 #endif /* VIDEO_VIDEO_DRIVER_HPP */