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/common.h

@@ -227,6 +227,9 @@ typedef struct session {
 	bool first_frame;
 	/// Whether screen has been turned off
 	bool screen_is_off;
+	/// We asked X server to send us a event for the end of a vblank, and we haven't
+	/// received one yet.
+	bool vblank_event_requested;
 	/// Event context for X Present extension.
 	uint32_t present_event_id;
 	xcb_special_event_t *present_event;