Cocoa: Fix EGL support

This adds support for using GLFW with EGL backends on macOS.  While EGL
isn't available by default on macOS, there are third-party
implementations like SwiftShader.

The addition of MoltenVK support added an override to makeBackingLayer.
In non-Vulkan mode, this means that we are returning nil from that
function.  The NSGL implementation creates this layer manually later on,
but other OpenGL backends don't necessarily do that.

Closes #985.

src/cocoa_window.m

@@ -423,6 +423,9 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
 
 - (id)makeBackingLayer
 {
+    if (!window->ns.layer) {
+        window->ns.layer = [super makeBackingLayer];
+    }
     return window->ns.layer;
 }