Fix error when switching to fullscreen

This error only happens after pause screen opened once.
Changes to MTGS.cpp fix an unrelated logic error I noticed, seemingly benign.

Author: pips3

pcsx2/ImGui/ImGuiManager.cpp

@@ -250,11 +250,14 @@ void ImGuiManager::UpdateScale()
 	s_global_scale = scale;
 	SetStyle();
 
+	// can't do this in the middle of a frame
+	ImGui::EndFrame();
 	if (!AddImGuiFonts(HasFullscreenFonts()))
 		pxFailRel("Failed to create ImGui font text");
 
 	if (!g_gs_device->UpdateImGuiFontTexture())
 		pxFailRel("Failed to recreate font texture after scale+resize");
+	NewFrame();
 }
 
 void ImGuiManager::NewFrame()

pcsx2/MTGS.cpp

@@ -962,9 +962,11 @@ void MTGS::ResizeDisplayWindow(int width, int height, float scale)
 
 		// If we're paused, re-present the current frame at the new window size.
 		if (VMManager::GetState() == VMState::Paused)
+		{
 			GSPresentCurrentFrame();
 			//New drawrect calculated in GSPresentCurrentFrame(), so resize imgui
 			GSResizeImGUIOnly(false);
+		}
 
 	});
 }