Cocoa: Fix clearing of unrelated window style bits

Whenever GLFW changed the window style mask, a new mask was created
from scratch based on the attributes set on the GLFW window object.
This caused us to potentially clear unrelated window style bits.

This was always wrong but became a critical issue when Cocoa began
throwing an exception if an application cleared the
NSWindowStyleMaskFullScreen while the window is in macOS fullscreen.

This commit reworks all style mask editing so it only changes the
relevant bits, preserving all others.

This is only a narrow bug fix to prevent crashes, intended for the
stable branch.  Our interaction with macOS fullscreen is still very
poor.  The next step after this is a set of patches that improve the
interaction between the current API and macOS fullscreen.

Fixes #1886
Fixes #2110

(cherry picked from commit 0d599026d0)

README.md

@@ -137,6 +137,10 @@ information on what to include when reporting a bug.
  - [Cocoa] Bugfix: A connected Apple AirPlay would emit a useless error (#1791)
  - [Cocoa] Bugfix: The EGL and OSMesa libraries were not unloaded on termination
  - [Cocoa] Bugfix: `GLFW_MAXIMIZED` was always true when `GLFW_RESIZABLE` was false
+ - [Cocoa] Bugfix: Changing `GLFW_DECORATED` in macOS fullscreen would abort
+   application (#1886)
+ - [Cocoa] Bugfix: Setting a monitor from macOS fullscreen would abort
+   application (#2110)
  - [X11] Bugfix: The OSMesa libray was not unloaded on termination
  - [X11] Bugfix: A malformed response during selection transfer could cause a segfault
  - [X11] Bugfix: Some calls would reset Xlib to the default error handler (#2108)