Treat first frame after redirection differently

Because first frame no window has their pixmap bound, which doesn't
happen in frames after the first. If a window is unmapped in that frame,
the compositor will try to render a window with no pixmap bound if
fading is enabled.

Now we keep track if we are in the first frame, and if that's the case
we skip fading in unmap/destroy.

Fixes #239, bug number 2

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

src/common.h

@@ -210,6 +210,8 @@ typedef struct session {
 #endif
 	/// Sync fence to sync draw operations
 	xcb_sync_fence_t sync_fence;
+	/// Whether we are rendering the first frame after screen is redirected
+	bool first_frame;
 
 	// === Operation related ===
 	/// Flags related to the root window