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/x.h

@@ -137,6 +137,9 @@ x_print_error(unsigned long serial, uint8_t major, uint8_t minor, uint8_t error_
 xcb_pixmap_t
 x_create_pixmap(session_t *ps, uint8_t depth, xcb_drawable_t drawable, uint16_t width, uint16_t height);
 
+bool
+x_validate_pixmap(session_t *ps, xcb_pixmap_t pxmap);
+
 /**
  * Free a <code>winprop_t</code>.
  *