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/kernel.h

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) Yuxuan Shui <yshuiv7@gmail.com>
+
+#pragma once
+
+/// Code for generating convolution kernels
+
+typedef struct conv {
+  int size;
+  double data[];
+} conv;
+
+/// Calculate the sum of a rectangle part of the convolution kernel
+/// the rectangle is defined by top left (x, y), and a size (width x height)
+double
+sum_kernel(conv *map, int x, int y, int width, int height);
+
+/// Create a kernel with gaussian distribution of radius r
+conv *gaussian_kernel(double r);