Assess the mouse capture state when gaining window focus

This fixes mouse visibility issues on the Raspberry Pi,
which uses an older customized version of SDL2. SDL2 on
the Pi fires different windows events in a different
order, making dealing with states challenging.

To handle this generically, we additionally assess the
mouse capture state after the Window has gained focus.

We also add a "has_focus" state tracking boolean to the
SDL object, which protects against general SDL mouse
trigger events from changing the mouse state before the
Window has "risen" and delivered us an EXPOSED or FOCUS
event. In the prior code rapidly clicking the mouse while
DOSBox was starting could freeze the mouse cursor in
place on the Pi.

We still also assess the mouse state on EXPOSE events,
which is fired after full - windowed modes are toggled.
The EXPOSE even occurs reliably on OSX, X11, Wayland, and
Windows after the window has settled (but is never fired
on the Pi).

src/gui/sdlmain.cpp

@@ -310,11 +310,12 @@ struct SDL_Block {
 		SDL_PixelFormat *pixelFormat = nullptr;
 	} texture;
 	struct {
-		int xsensitivity;
-		int ysensitivity;
-		Bitu sensitivity;
-		MouseControlType control_choice;
-		bool middle_will_release;
+		int xsensitivity = 0;
+		int ysensitivity = 0;
+		int sensitivity = 0;
+		MouseControlType control_choice = Seamless;
+		bool middle_will_release = true;
+		bool has_focus = false;
 	} mouse;
 	int  ppscale_x, ppscale_y; /* x and y scales for pixel-perfect     */
 	bool double_h, double_w;   /* double-height and double-width flags */
@@ -1295,6 +1296,14 @@ void GFX_SetShader(const char* src) {
 }
 
 void GFX_ToggleMouseCapture(void) {
+	/*
+	 * Only process mouse events when we have focus.
+	 * This protects against out-of-order event issues such
+	 * as acting on clicks before the window is drawn.
+	 */
+	if (!sdl.mouse.has_focus)
+		return;
+	
 	assertm(sdl.mouse.control_choice != NoMouse,
 	        "SDL: Mouse capture is invalid when NoMouse is configured [Logic Bug]");
 
@@ -1313,30 +1322,50 @@ static void ToggleMouseCapture(bool pressed) {
 	GFX_ToggleMouseCapture();
 }
 
-void GFX_UpdateMouseAfterExposure(void) {
+/*  
+ *  Assesses the following:
+ *   - current window size (full or not),
+ *   - mouse capture state, (yes or no).
+ *   - desired capture type (start, click, seamless), and
+ *   - if we're starting up for the first time,
+ *  to determine if the mouse-capture state should be toggled.
+ *  Note that this also acts a filter: we don't want to repeatedly
+ *  re-apply the same mouse capture state over and over again, so most
+ *  of the time this function will (or should) decide to do nothing.
+ */
+void GFX_UpdateMouseState(void) {
+
+	// This function is only be run when the window is shown or has focus
+	sdl.mouse.has_focus = true;
 
 	// Used below
 	static bool has_run_once = false;
 
-	// We've switched to or started in fullscreen, so capture the mouse
-	// This is valid for all modes except for nomouse.
+	/*
+	 *  We've switched to or started in fullscreen, so capture the mouse
+	 *  This is valid for all modes except for nomouse.
+	 */
 	if (sdl.desktop.fullscreen
 	    && !mouse_is_captured
 	    && sdl.mouse.control_choice != NoMouse) {
 		GFX_ToggleMouseCapture();
 
-	// If we've switched-back from fullscreen, then released the mouse
-	// if it's captured and in seamless-mode.
+	/*
+	 * If we've switched-back from fullscreen, then release the mouse
+	 * if it's captured and in seamless-mode.
+	 */
 	} else if (!sdl.desktop.fullscreen
 	           && mouse_is_captured
 	           && sdl.mouse.control_choice == Seamless) {
 			GFX_ToggleMouseCapture();
 			SDL_ShowCursor(SDL_DISABLE);
 
-	// If none of the above are true /and/ has_run_once is false,
-	// then we're starting up the first time, so we:
-	//  - Capture the mouse if configured onstart is set.
-	//  - Hide the mouse if seamless or nomouse are set.
+	/*
+	 *  If none of the above are true /and/ we're starting
+	 *  up the first time, then:
+	 *  - Capture the mouse if configured onstart is set.
+	 *  - Hide the mouse if seamless or nomouse are set.
+	 */
 	} else if (!has_run_once) {
 		if (sdl.mouse.control_choice == CaptureOnStart) {
 			SDL_RaiseWindow(sdl.window);
@@ -2179,13 +2208,16 @@ void GFX_Events() {
 				case SDL_WINDOWEVENT_RESIZED:
 					GFX_HandleVideoResize(event.window.data1, event.window.data2);
 					continue;
+				// window has been exposed and needs to be redrawn
 				case SDL_WINDOWEVENT_EXPOSED:
-					if (sdl.draw.callback) sdl.draw.callback( GFX_CallBackRedraw );
-					GFX_UpdateMouseAfterExposure();
+					if (sdl.draw.callback)
+						sdl.draw.callback(GFX_CallBackRedraw);
+					GFX_UpdateMouseState();
 					continue;
 				case SDL_WINDOWEVENT_FOCUS_GAINED:
 					SetPriority(sdl.priority.focus);
 					CPU_Disable_SkipAutoAdjust();
+					GFX_UpdateMouseState();
 					break;
 				case SDL_WINDOWEVENT_FOCUS_LOST:
 #ifdef WIN32
@@ -2197,6 +2229,7 @@ void GFX_Events() {
 					SetPriority(sdl.priority.nofocus);
 					GFX_LosingFocus();
 					CPU_Enable_SkipAutoAdjust();
+					sdl.mouse.has_focus = false;
 					break;
 				default: ;
 			}