Update screenshots

It's been five years since the previous xC screenshot was made.

README.adoc

@@ -13,7 +13,7 @@ The IRC client, and the core of 'xK'.  It is largely defined by building on top
 of GNU Readline or BSD Editline that have been hacked to death.  Its interface
 should feel somewhat familiar for weechat or irssi users.
 
-image::xC.png[align="center"]
+image::xC.webp[align="center"]
 
 It has most features you'd expect of an IRC client, such as being multiserver,
 a powerful configuration system, integrated help, text formatting, automatic
@@ -30,6 +30,8 @@ It intentionally differs in that it uses a sans-serif font, and it shows
 the list of all buffers in a side panel.  Otherwise it is a near replica,
 including keyboard shortcuts.
 
+image::xP.webp[align="center"]
+
 xD
 --
 The IRC daemon.  It is designed for use as a regular user application rather