GS: Snapshot the display resolution for cross-thread reads

GSgetDisplayResolution() took a raw pointer from g_gs_renderer and
dereferenced it, but guncon2 calls it from the EE thread. If the GS
thread reopened the renderer between the null check and the read, the
pointer dangled.

Store the resolution on the GS thread where it is already computed and
read the copy instead. This matches s_last_draw_rect directly above it,
which exists for this exact reason and which the same guncon2 call site
already reads via GSTranslateWindowToDisplayCoordinates.

Reported by chaoticgd.

Author: dyzt

pcsx2/GS/GS.cpp

@@ -662,21 +662,6 @@ void GSgetInternalResolution(int* width, int* height)
 	*height = res.y;
 }
 
-void GSgetDisplayResolution(int* width, int* height)
-{
-	GSRenderer* gs = g_gs_renderer.get();
-	if (!gs)
-	{
-		*width = 0;
-		*height = 0;
-		return;
-	}
-
-	const GSVector2i res(gs->PCRTCDisplays.GetResolution());
-	*width = res.x;
-	*height = res.y;
-}
-
 void GSgetStats(SmallStringBase& info)
 {
 	GSPerfMon& pm = g_perfmon;

pcsx2/GS/Renderers/Common/GSRenderer.cpp

@@ -46,6 +46,7 @@ std::unique_ptr<GSRenderer> g_gs_renderer;
 // Since we read this on the EE thread, we can't put it in the renderer, because
 // we might be switching while the other thread reads it.
 static GSVector4 s_last_draw_rect;
+static GSVector2i s_last_display_resolution;
 
 // Last time we reset the renderer due to a GPU crash, if any.
 static Common::Timer::Value s_last_gpu_reset_time;
@@ -57,6 +58,7 @@ GSRenderer::GSRenderer()
 	: m_shader_time_start(Common::Timer::GetCurrentValue())
 {
 	s_last_draw_rect = GSVector4::zero();
+	s_last_display_resolution = GSVector2i(0, 0);
 }
 
 GSRenderer::~GSRenderer() = default;
@@ -221,6 +223,7 @@ bool GSRenderer::Merge(int field)
 	}
 
 	const GSVector2i resolution = PCRTCDisplays.GetResolution();
+	s_last_display_resolution = resolution;
 	fs = GSVector2i(static_cast<int>(static_cast<float>(resolution.x) * GetUpscaleMultiplier()),
 		static_cast<int>(static_cast<float>(resolution.y) * GetUpscaleMultiplier()));
 
@@ -997,6 +1000,12 @@ void GSTranslateWindowToDisplayCoordinates(float window_x, float window_y, float
 	*display_y = rel_y / draw_height;
 }
 
+void GSgetDisplayResolution(int* width, int* height)
+{
+	*width = s_last_display_resolution.x;
+	*height = s_last_display_resolution.y;
+}
+
 void GSSetDisplayAlignment(GSDisplayAlignment alignment)
 {
 	s_display_alignment = alignment;