Add GLFW_CURSOR_HIDDEN.

Uses addCursorRect:cursor: as discussed.. well, too long ago. Anyhow,
this will effectively hide the cursor while it is inside a window with
GLFW_CURSOR_HIDDEN or GLFW_CURSOR_CAPTURED enabled. This shouldn't mess
up cursor retain counts either, unlike previous uses of the hide/unhide
methods on NSCursor.

It does allocate a small, single-pixel image for an invisible cursor,
as well as the cursor itself, but that shouldn't be too much trouble.

src/cocoa_window.m

@@ -309,6 +309,8 @@ static int convertMacKeyCode(unsigned int macKeyCode)
 // Content view class for the GLFW window
 //------------------------------------------------------------------------
 
+static NSCursor *emptyCursor = nil;
+
 @interface GLFWContentView : NSView
 {
     _GLFWwindow* window;
@@ -321,6 +323,16 @@ static int convertMacKeyCode(unsigned int macKeyCode)
 
 @implementation GLFWContentView
 
++ (void)initialize
+{
+    if (self == [GLFWContentView class])
+    {
+        NSImage *emptyImage = [[NSImage alloc] initWithSize:NSMakeSize(1, 1)];
+        emptyCursor = [[NSCursor alloc] initWithImage:emptyImage hotSpot:NSZeroPoint];
+        [emptyImage release];
+    }
+}
+
 - (id)initWithGlfwWindow:(_GLFWwindow *)initWindow
 {
     self = [super init];
@@ -494,6 +506,11 @@ static int convertMacKeyCode(unsigned int macKeyCode)
         _glfwInputScroll(window, deltaX, deltaY);
 }
 
+- (void)resetCursorRects
+{
+    [self addCursorRect:[self bounds] cursor:emptyCursor];
+}
+
 @end
 
 
@@ -929,16 +946,21 @@ void _glfwPlatformSetCursorPos(_GLFWwindow* window, double x, double y)
 
 void _glfwPlatformSetCursorMode(_GLFWwindow* window, int mode)
 {
+    // Unhide the cursor if the last mode was CAPTURED.
+    if (window->cursorMode == GLFW_CURSOR_CAPTURED) {
+        CGAssociateMouseAndMouseCursorPosition(true);
+    }
+
     switch (mode)
     {
         case GLFW_CURSOR_NORMAL:
-            [NSCursor unhide];
-            CGAssociateMouseAndMouseCursorPosition(true);
+            [window->ns.object disableCursorRects];
             break;
         case GLFW_CURSOR_HIDDEN:
+            [window->ns.object enableCursorRects];
             break;
         case GLFW_CURSOR_CAPTURED:
-            [NSCursor hide];
+            [window->ns.object enableCursorRects];
             CGAssociateMouseAndMouseCursorPosition(false);
             break;
     }