sokol: stop drawing when there's nothing to redraw

this removes cpu load when idle, and reduces it when moving the
mouse around, etc.
the implementation reflects the landscape of the magical world
of linux; xlib is not thread-safe so we can't push a dummy event,
and select can mark an fd ready even though io would block.

based on ideas from floooh/sokol#640

lib/sokol_app.h

@@ -1825,6 +1825,10 @@ SOKOL_APP_API_DECL sapp_desc sapp_query_desc(void);
 SOKOL_APP_API_DECL void sapp_request_quit(void);
 /* cancel a pending quit (when SAPP_EVENTTYPE_QUIT_REQUESTED has been received) */
 SOKOL_APP_API_DECL void sapp_cancel_quit(void);
+/* interrupt waiting for event */
+SOKOL_APP_API_DECL void sapp_wakethefup(void);
+/* this causes the app to block until input is received. the flag will be cleared after that, so you will have to call it again if necessary. */
+SOKOL_APP_API_DECL void sapp_input_wait(bool set);
 /* initiate a "hard quit" (quit application without sending SAPP_EVENTTYPE_QUIT_REQUESTED) */
 SOKOL_APP_API_DECL void sapp_quit(void);
 /* call from inside event callback to consume the current event (don't forward to platform) */
@@ -2164,6 +2168,7 @@ inline void sapp_run(const sapp_desc& desc) { return sapp_run(&desc); }
     #include <pthread.h>    /* only used a linker-guard, search for _sapp_linux_run() and see first comment */
     #include <time.h>
     #include <poll.h>
+    #include <sys/select.h>
 #endif
 
 #if defined(_SAPP_APPLE)
@@ -2675,7 +2680,7 @@ typedef struct {
 #define GLX_RGBA_BIT 0x00000001
 #define GLX_WINDOW_BIT 0x00000001
 #define GLX_DRAWABLE_TYPE 0x8010
-#define GLX_RENDER_TYPE	0x8011
+#define GLX_RENDER_TYPE    0x8011
 #define GLX_DOUBLEBUFFER 5
 #define GLX_RED_SIZE 8
 #define GLX_GREEN_SIZE 9
@@ -2863,6 +2868,7 @@ typedef struct {
     bool event_consumed;
     bool html5_ask_leave_site;
     bool onscreen_keyboard_shown;
+    bool input_wait;
     int window_width;
     int window_height;
     int framebuffer_width;
@@ -11227,6 +11233,34 @@ _SOKOL_PRIVATE void _sapp_egl_destroy(void) {
 
 #endif /* _SAPP_GLX */
 
+/* disgusting; x11 is not thread-safe so we can't send dummy events,
+ * forcing us to do this kind of shit with poll or select if we want
+ * to wait for next event but react to custom events to stop */
+_SOKOL_PRIVATE void _sapp_x11_wait_event(void) {
+    if (!_sapp.input_wait) {
+        return;
+    }
+    int cfd = ConnectionNumber(_sapp.x11.display);
+    fd_set fds;
+    FD_ZERO(&fds);
+    FD_SET(cfd, &fds);
+    /* in the magical world of linux, select may return ready even
+     * though a read would block */
+    do{
+        if (select(cfd+1, &fds, NULL, NULL, NULL) < 0) {
+            break;
+        }
+    }while(!XPending(_sapp.x11.display));
+}
+
+_SOKOL_PRIVATE void _sapp_x11_wakethefup(void) {
+    if (!_sapp.x11.display || !_sapp.input_wait) {
+        return;
+    }
+    int cfd = ConnectionNumber(_sapp.x11.display);
+    write(cfd, &cfd, 0);
+}
+
 _SOKOL_PRIVATE void _sapp_linux_run(const sapp_desc* desc) {
     /* The following lines are here to trigger a linker error instead of an
         obscure runtime error if the user has forgotten to add -pthread to
@@ -11273,6 +11307,7 @@ _SOKOL_PRIVATE void _sapp_linux_run(const sapp_desc* desc) {
     XFlush(_sapp.x11.display);
     while (!_sapp.quit_ordered) {
         _sapp_timing_measure(&_sapp.timing);
+        _sapp_x11_wait_event();
         int count = XPending(_sapp.x11.display);
         while (count--) {
             XEvent event;
@@ -11547,6 +11582,16 @@ SOKOL_API_IMPL void sapp_quit(void) {
     _sapp.quit_ordered = true;
 }
 
+SOKOL_API_IMPL void sapp_wakethefup(void) {
+    #if defined(_SAPP_LINUX)
+        _sapp_x11_wakethefup();
+    #endif
+}
+
+SOKOL_API_IMPL void sapp_input_wait(bool set) {
+    _sapp.input_wait = set;
+}
+
 SOKOL_API_IMPL void sapp_consume_event(void) {
     _sapp.event_consumed = true;
 }

sokol/draw.c

@@ -327,11 +327,16 @@ drawproc(void *)
 		if((req = recvul(drawc)) == 0)
 			break;
 		reqs |= req;
-		if((req & Reqredraw) != 0)
+		if((req & Reqredraw) != 0){
 			stop = 0;
+			sapp_input_wait(false);
+		}
 		if(!stop && (req & (Reqrefresh | Reqshape)) != 0){
-			if(!redraw())
+			if(!redraw()){
 				stop = 1;
+				sapp_input_wait(true);
+			}else
+				sapp_wakethefup();
 		}
 	}
 }
@@ -344,13 +349,16 @@ frame(void)
 
 	CLK0(fclk);
 	ctx = snk_new_frame();
-	if((reqs & Reqresetdraw) != 0)
+	if((reqs & Reqresetdraw) != 0){
+		reqs &= ~Reqresetdraw;
 		resize();
+	}
 	if((reqs & Reqresetui) != 0){
+		reqs &= ~Reqresetui;
 		resetui();
 		updateview();
 	}
-	reqs &= ~(Reqresetui | Reqresetdraw);
+	reqs &= ~(Reqrefresh | Reqshape | Reqshallowdraw | Reqredraw);
 	drawui(ctx);
 	CLK0(clk);
 	flush();