[client] ds: refactor common EGL swap with damage logic

This commit creates a new utility library, eglutil.h, which contains code
to detect and use EGL_KHR_swap_buffers_with_damage or its EXT equivalent.

This logic used to be duplicated between the X11 and Wayland display servers,
which is not ideal.

client/CMakeLists.txt

@@ -119,6 +119,7 @@ set(SOURCES
 	src/clipboard.c
 	src/kb.c
 	src/egl_dynprocs.c
+	src/eglutil.c
 )
 
 # Force cimgui to build as a static library.