core: don't request vblank events when we are not rendering

Previously everytime we receive a vblank event, we always request a new
one. This made the logic somewhat simpler. But this generated many
useless vblank events, and wasted power. We only need vblank events for
two things:

1. after we rendered a frame, we need to know when it has been displayed
   on the screen.
2. estimating the refresh rate.

This commit makes sure we only request vblank events when it's actually
needed.

Fixes #1079

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/x.c

@@ -10,6 +10,7 @@
 #include <xcb/composite.h>
 #include <xcb/damage.h>
 #include <xcb/glx.h>
+#include <xcb/present.h>
 #include <xcb/randr.h>
 #include <xcb/render.h>
 #include <xcb/sync.h>
@@ -777,6 +778,17 @@ err:
 	return false;
 }
 
+void x_request_vblank_event(session_t *ps, uint64_t msc) {
+	if (ps->vblank_event_requested) {
+		return;
+	}
+
+	auto cookie =
+	    xcb_present_notify_msc(ps->c.c, session_get_target_window(ps), 0, msc, 0, 0);
+	set_cant_fail_cookie(&ps->c, cookie);
+	ps->vblank_event_requested = true;
+}
+
 /**
  * Convert a struct conv to a X picture convolution filter, normalizing the kernel
  * in the process. Allow the caller to specify the element at the center of the kernel,