libXcomposite 0.4.6

• Build xz tarballs instead of bzip2
• Fix spelling/wording issues
• gitlab CI: add a basic build test
• Mark two dpy parameters const as suggested by cppcheck
• Remove unnecessary casts from malloc() and free() calls
• Wrap XComposite*CheckExtension() in do { ... } while(0)
• Handle implicit conversion warnings from clang
• XCompositeCloseDisplay: Mark codes as unused