Cocoa: Replace global autorelease pool with blocks
This is another small step towards having GLFW play nice with other toolkits sharing the same process, including AppKit. Any macOS platform function that touches Cocoa must now wrap itself in an autoreleasepool block. Since GLFW no longer provides an autoreleasepool outside of its functions, THIS MAY BREAK EXISTING CODE MIXING GLFW AND COCOA. Sorry! Please add your own autoreleasepool blocks as needed. Fixes #1107. Closes #1114.