Add input and bounding shapes support (#2742)

Basic idea: if the window has a shape, set the parent container shape as
the union of the window shape and the shape of the frame borders.

Co-authored-by: Uli Schlachter <psychon@znc.in>

include/handlers.h

@@ -16,6 +16,7 @@
 
 extern int randr_base;
 extern int xkb_base;
+extern int shape_base;
 
 /**
  * Adds the given sequence to the list of events which are ignored.