treewide: drop COMPOSITE symbol

It's always enabled for very long time now (at least since meson transition),
there doesn't seem to be any need to ever disable it again. So we can reduce
code complexity by removing all the ifdef's.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

include/windowstr.h

@@ -160,10 +160,8 @@ typedef struct _Window {
     unsigned redirectDraw:2;    /* COMPOSITE rendering redirect */
     unsigned forcedBG:1;        /* must have an opaque background */
     unsigned unhittable:1;      /* doesn't hit-test, for rootless */
-#ifdef COMPOSITE
     unsigned damagedDescendants:1;      /* some descendants are damaged */
     unsigned inhibitBGPaint:1;  /* paint the background? */
-#endif
 
     PropertyPtr properties;     /* default: NULL */
 } WindowRec;