Cocoa: Implement GLFW_TRANSPARENT

This is an extract of a commit, minimally edited to ensure it compiles.

Closes #663.
Related to #197.

src/cocoa_window.m

@@ -413,7 +413,11 @@ static const NSRange kEmptyRange = { NSNotFound, 0 };
 
 - (BOOL)isOpaque
 {
-    return YES;
+    // Set to NO even if alphaMask is not used;
+    //   The NSView/GLFWContentView does not need to be opaque anyway,
+    //   and to avoid keeping track of alphaMask inside the NSView we
+    //   just return NO here instead.
+    return NO;
 }
 
 - (BOOL)canBecomeKeyView
@@ -1081,6 +1085,12 @@ static GLFWbool createNativeWindow(_GLFWwindow* window,
     if (wndconfig->ns.retina)
         [window->ns.view setWantsBestResolutionOpenGLSurface:YES];
 
+    if (window->transparent)
+    {
+        [window->ns.object setOpaque:NO];
+        [window->ns.object setBackgroundColor:[NSColor clearColor]];
+    }
+
     [window->ns.object setContentView:window->ns.view];
     [window->ns.object makeFirstResponder:window->ns.view];
     [window->ns.object setTitle:[NSString stringWithUTF8String:wndconfig->title]];

src/nsgl_context.m

@@ -296,6 +296,12 @@ GLFWbool _glfwCreateContextNSGL(_GLFWwindow* window,
         return GLFW_FALSE;
     }
 
+    if (window->transparent)
+    {
+        GLint opaque = 0;
+        [window->context.nsgl.object setValues:&opaque forParameter:NSOpenGLCPSurfaceOpacity];
+    }
+
     [window->context.nsgl.object setView:window->ns.view];
 
     window->context.makeCurrent = makeContextCurrentNSGL;