libretro: vulkan: wait before ticking (#2004)

Ensure the scheduler worker thread has finished processing all dispatched command chunks before the rasterizer cache's garbage collector destroys sentenced surfaces.

src/video_core/renderer_vulkan/vk_rasterizer.cpp

@@ -140,6 +140,7 @@ RasterizerVulkan::RasterizerVulkan(Memory::MemorySystem& memory, Pica::PicaCore&
 RasterizerVulkan::~RasterizerVulkan() = default;
 
 void RasterizerVulkan::TickFrame() {
+    scheduler.WaitWorker();
     res_cache.TickFrame();
 }