compositor: Switch from compton to picom

Switch to ibhagwan's picom fork with dual_kawase blur and rounded
corners.

.config/compositor

@@ -1 +1 @@
-./compton
+picom

.config/compton/compton.conf

@@ -1,4 +0,0 @@
-backend = "glx";
-blur-method = "kawase";
-blur-strength = 10;
-blur-background = true;

.config/picom/picom.conf

@@ -0,0 +1,11 @@
+experimental-backends = true;
+backend = "glx";
+
+corner-radius = 20;
+
+blur:
+{
+	method = "kawase";
+	strength = 10;
+	background = true;
+}