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/render.h

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) Yuxuan Shui <yshuiv7@gmail.com>
+#pragma once
+
+#include <xcb/render.h>
+#include "region.h"
+
+typedef struct _glx_texture glx_texture_t;
+typedef struct glx_prog_main glx_prog_main_t;
+typedef struct win win;
+typedef struct session session_t;
+
+typedef struct paint {
+  xcb_pixmap_t pixmap;
+  xcb_render_picture_t pict;
+  glx_texture_t *ptex;
+} paint_t;
+
+void
+render(session_t *ps, int x, int y, int dx, int dy, int wid, int hei,
+    double opacity, bool argb, bool neg,
+    xcb_render_picture_t pict, glx_texture_t *ptex,
+    const region_t *reg_paint, const glx_prog_main_t *pprogram);
+void
+paint_one(session_t *ps, win *w, const region_t *reg_paint);
+
+void
+paint_all(session_t *ps, region_t *region, const region_t *region_real, win * const t);
+
+void free_picture(xcb_connection_t *c, xcb_render_picture_t *p);
+
+void free_paint(session_t *ps, paint_t *ppaint);