picom: tentative to use a shader for rounded borders

This does not work with i3 border.

dotfiles/picom.conf

@@ -67,3 +67,6 @@ shadow-exclude = [
   "name = 'Screen Sharing Tracker'" # Jitsi
 ];
 fade-exclude = [ ];
+
+corner-radius = @SHADOW_RADIUS@
+window-shader-fg = "rounded-borders.glsl"

dotfiles/rounded-borders.glsl

@@ -0,0 +1,42 @@
+#version 330
+
+in vec2 texcoord;
+uniform sampler2D tex;
+uniform float corner_radius;
+
+vec4 default_post_processing(vec4 c);
+
+vec4 add_rounded_corners(
+    vec4 win_color,
+    vec2 tex_coord,
+    vec2 tex_size,
+    float radius,
+    float thickness)
+{       
+    // If we're far from corners, we just pass window texels through
+    vec2 corner_distance = min(abs(tex_coord), abs(tex_size - 1 - tex_coord));
+    if (any(greaterThan(corner_distance, vec2(radius))))
+        return win_color;
+
+    // Distance from the closest arc center
+    vec2 center_distance = min(
+        abs(vec2(radius) - tex_coord),
+        abs(vec2(tex_size - radius) - tex_coord));
+
+    // Do some simple anti-aliasing
+    float inner_radius = radius - thickness;
+    float feather = 1 / inner_radius;
+    float r = length(center_distance) / inner_radius;
+    float blend = smoothstep(1, 1 + feather, r);
+    
+    vec4 border_color = texture2D(tex, vec2(0), 0);
+    return blend * border_color + (1.0 - blend) * win_color;
+}
+
+vec4 window_shader()
+{
+    vec2 tex_size = textureSize(tex, 0);
+    vec4 c = texture2D(tex, texcoord / tex_size, 0);
+    vec4 with_corners = add_rounded_corners(c, texcoord, tex_size, corner_radius, 6);
+    return default_post_processing(with_corners);    
+}