egl: Implement EGL_KHR_debug (v2)

Wire up the debug entrypoints to EGL dispatch, and add the extension
string to the client extension list.

v2:
- Lots of style fixes
- Fix missing EGLAPIENTRYs
- Factor out valid attribute check
- Lock display in eglLabelObjectKHR as needed, and use RETURN_EGL_*
- Move "EGL_KHR_debug" into asciibetical order in client extension
  string

Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Emil Velikov <emil.veliko@collabora.com>

src/egl/main/eglglobals.c

@@ -56,6 +56,7 @@ struct _egl_global _eglGlobal =
    " EGL_EXT_platform_wayland"
    " EGL_EXT_platform_x11"
    " EGL_KHR_client_get_all_proc_addresses"
+   " EGL_KHR_debug"
    " EGL_MESA_platform_gbm",
 
    NULL, /* debugCallback */