Move rendering related functions out of compton.c

Moved:
* Blur kernel related functions to kernel.c
* Vsync related functions to vsync.c
* paint related functions to render.c

This will make the `split-backend` branch easier to rebase.

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

src/vsync.h

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) Yuxuan Shui <yshuiv7@gmail.com>
+#include <stdbool.h>
+
+typedef struct session session_t;
+
+bool vsync_init(session_t *ps);
+void vsync_wait(session_t *ps);
+void vsync_deinit(session_t *ps);