Integrated Chrome tracing

Added support for chrome tracing by integrating the
minitrace library (github.com/hrydgard/minitrace),
and modified it with a background flushing thread that allows
tracing to continue automatically after the predefined
1 million event limit.

Menu items and an accelerator (Ctr+T) have also been
added. Starting and stopping the trace simply replaces
trace.json with the new trace data.

The feature is disabled by default. Pass MINITRACE=y
to the build command to enable it. Some traces are
already added as an example, however they won't have
any effect if the feature is disabled.

src/include/86box/plat.h

@@ -66,6 +66,11 @@ extern int	dopause,			/* system is paused */
 		doresize,			/* screen resize requested */
 		quited,				/* system exit requested */
 		mouse_capture;			/* mouse is captured in app */
+
+#ifdef MTR_ENABLED
+extern int tracing_on;
+#endif
+
 extern uint64_t	timer_freq;
 extern int	infocus;
 extern char	emu_version[200];		/* version ID string */
@@ -165,6 +170,11 @@ extern void	startblit(void);
 extern void	endblit(void);
 extern void	take_screenshot(void);
 
+#ifdef MTR_ENABLED
+extern void init_trace(void);
+extern void shutdown_trace(void);
+#endif
+
 #ifdef __cplusplus
 }
 #endif

src/include/86box/resource.h

@@ -287,6 +287,11 @@
 #define IDM_ACTION_EXIT		40014
 #define IDM_ACTION_CTRL_ALT_ESC 40015
 #define IDM_ACTION_PAUSE	40016
+#ifdef MTR_ENABLED
+#define IDM_ACTION_BEGIN_TRACE	40017
+#define IDM_ACTION_END_TRACE	40018
+#define IDM_ACTION_TRACE        40019
+#endif
 #define IDM_CONFIG		40020
 #define IDM_CONFIG_LOAD		40021
 #define IDM_CONFIG_SAVE		40022