readme: update and cleanup xcb deps

Switch from the pkg-config dependency names to the upstream
libxcb repository names. Each repository contains multiple
libraries but is generally available in distributions as a single
package.

libxcb contains xcb, xcb-composite, xcb-xfixes, xcb-xinput and
xcb-shm. libxcb-render-util contains xcb-render. libxcb-wm
contains xcb-icccm. xcb-image was outdated and is no longer needed.

README.md

@@ -55,15 +55,10 @@ Install dependencies:
 If you choose to enable X11 support:
 
 * xwayland (build-time only, optional at runtime)
-* xcb
+* libxcb
-* xcb-composite
+* libxcb-render-util
-* xcb-xfixes
+* libxcb-wm
-* xcb-xinput
+* libxcb-errors (optional, for improved error reporting)
-* xcb-icccm
-* xcb-image
-* xcb-render
-* xcb-shm
-* xcb-errors (optional, for improved error reporting)
 
 Run these commands: