GPU: Rasterize software depth on a worker thread

The depth draws were queued and rasterized on the emu thread at each render target change, in
Wipeout about 40 small batches a frame, and around 40% of that thread's time. Now a render target
change hands the batch to a worker thread and carries on. Waiting for it is left to whatever reads
the depth buffers: DrawSync and ListSync, block transfers, framebuffer copies, showing a frame,
savestates and shutdown, and the texture cache when the texture or CLUT it reads overlaps a depth
buffer still being written.

Batches grow as needed instead of being preallocated at full size. A full batch is now handed over
and a new one started, where the draw used to be dropped.

Wipeout headless, output unchanged: 16% less wall time on a Galaxy S8 (8.2 s to 6.9 s for 10
emulated seconds). On an M1, where headless waits on the GPU, the emu thread uses 38% less CPU time.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Core/System.cpp

@@ -652,6 +652,11 @@ void CPU_Shutdown(bool success) {
 	// Core_LockAgainstShutdown(); it's recursive, so the nested acquire in Memory::Shutdown() is fine.
 	CoreShutdownLock coreLock = Core_LockAgainstShutdown();
 
+	// The GPU outlives memory (GPU_Shutdown comes later), so it must stop writing to it now.
+	if (gpu) {
+		gpu->FlushPendingDrawing();
+	}
+
 	UninstallExceptionHandler();
 
 	GPURecord::Replay_Unload();