Fixes missing window preview on Alt+Tab

There was a crash on Alt+Tab in fullscreen for some specific driver:

https://code.wildfiregames.com/D1212

It was related only to post processing so it's time to try to enable
rendering again (but with provided option to disable it). Also without
rendering we might have some bugs in other components. See:

https://github.com/libsdl-org/SDL/issues/5033#issuecomment-981819661

The issue is reported here (first one):

https://wildfiregames.com/forum/topic/129061-two-vulkan-issues-in-a27-and-unrelated-choppiness-caused-by-control-groups/

binaries/data/config/default.cfg

@@ -107,6 +107,10 @@ shadowscovermap = false
 renderer.scale = 1.0
 renderer.upscale.technique = "fsr"
 
+; Allows to disable rendering when the game window is out of focus. It might
+; save some performance or fix old driver crashes on out of focus.
+renderer.renderwhenoutoffocus = "true"
+
 vsync = false
 particles = true
 fog = true

binaries/data/mods/public/gui/options/options.json

@@ -174,6 +174,12 @@
 				"label": "Mouse grab in window mode",
 				"tooltip": "Constrain mouse in the window mode to the window boundaries.",
 				"config": "window.mousegrabinwindowmode"
+			},
+			{
+				"type": "boolean",
+				"label": "Render out of focus",
+				"tooltip": "To save GPU workload, disable rendering when the game window is out of focus.",
+				"config": "renderer.renderwhenoutoffocus"
 			}
 		]
 	},

source/renderer/Renderer.cpp

@@ -508,7 +508,9 @@ void CRenderer::Resize(int width, int height)
 
 bool CRenderer::ShouldRender() const
 {
-	return !g_app_minimized && (g_app_has_focus || !g_VideoMode.IsInFullscreen());
+	const bool shouldRenderWhenOutOfFocus{
+		g_ConfigDB.Get("renderer.renderwhenoutoffocus", true)};
+	return !g_app_minimized && (g_app_has_focus || shouldRenderWhenOutOfFocus);
 }
 
 void CRenderer::RenderFrame(const bool needsPresent)