Protect savestates while the threaded software renderer is running (#1864)

* First crack at ensuring the render thread doesn't touch GPU state while it's being serialized

* Get rid of the semaphore wait

* Add some extra fields into GPU3D's serialization

* Oops, TempVertexBuffer is already serialized

* Move vertex serialization into its own method

* Lock the GPU3D state when rendering on the render thread or serializing it

* Revert "Lock the GPU3D state when rendering on the render thread or serializing it"

This reverts commit 2f49a551c1.

* Add comments that describe the synchronization within GPU3D_Soft

- I need to understand it before I can solve my actual problem
- Now I do

* Revert "Revert "Lock the GPU3D state when rendering on the render thread or serializing it""

This reverts commit 1977566a6d.

* Let's try locking the GPU3D state throughout NDS::RunFrame

- Just to see what happens

* Slim down the lock's scope

* Narrow the lock's scope some more

* Remove the lock entirely

* Try protecting the GPU3D state with just a mutex

- I'll clean this up once I know it works

* Remove a duplicate method definition

* Add a missing `noexcept` specifier

* Remove an unused function

* Cut some non-hardware state from `GPU3D`'s savestate

* Assume that the next frame after loading a savestate won't be identical

* Actually, it _is_ worth it

* Don't serialize the clip matrix

- It's recalculated anyway

* Serialize `RenderPolygonRAM` as an array of indexes

* Clean up some comments

- I liked the dialogue style, but oh well

* Try restarting the render thread instead of using the lock

- Let's see what happens

* Put the lock back

* Fix some polygon and vertex indexes being saved incorrectly

- Taking the difference between two pointers results in the number of elements, not the number of bytes

* Remove `SoftRenderer::StateBusy` since it turns out we don't need it

- The real synchronization was the friends we made along the way

src/GPU3D.h

@@ -47,6 +47,7 @@ struct Vertex
     // TODO maybe: hi-res color? (that survives clipping)
     s32 HiresPosition[2];
 
+    void DoSavestate(Savestate* file) noexcept;
 };
 
 struct Polygon
@@ -78,6 +79,7 @@ struct Polygon
 
     u32 SortKey;
 
+    void DoSavestate(Savestate* file) noexcept;
 };
 
 class Renderer3D;
@@ -269,7 +271,7 @@ public:
     u32 RenderClearAttr1 = 0;
     u32 RenderClearAttr2 = 0;
 
-    bool RenderFrameIdentical = false;
+    bool RenderFrameIdentical = false; // not part of the hardware state, don't serialize
 
     bool AbortFrame = false;
 
@@ -323,7 +325,7 @@ public:
 
     u32 FlushRequest = 0;
     u32 FlushAttributes = 0;
-    u32 ScrolledLine[256];
+    u32 ScrolledLine[256]; // not part of the hardware state, don't serialize
 };
 
 class Renderer3D