Use atomic functions where appropriate

Also, rename atomic functions to be consistent with the rest of the
platform/threading functions, and move atomic functions to threading*
files rather than platform* files

libobs/graphics/graphics-internal.h

@@ -241,5 +241,5 @@ struct graphics_subsystem {
 	DARRAY(struct vec2)    texverts[16];
 
 	pthread_mutex_t        mutex;
-	volatile int           ref;
+	volatile long          ref;
 };

libobs/graphics/graphics.c

@@ -179,13 +179,13 @@ void gs_entercontext(graphics_t graphics)
 		thread_graphics = graphics;
 	}
 
-	graphics->ref++;
+	os_atomic_inc_long(&graphics->ref);
 }
 
 void gs_leavecontext(void)
 {
 	if (thread_graphics) {
-		if (!--thread_graphics->ref) {
+		if (!os_atomic_dec_long(&thread_graphics->ref)) {
 			graphics_t graphics = thread_graphics;
 
 			graphics->exports.device_leavecontext(graphics->device);