Fix CGL multithreading error

Cargo.toml

@@ -38,4 +38,6 @@ winapi = "0.2"
 gdi32-sys = "0.2"
 user32-sys = "0.2"
 kernel32-sys = "0.2"
-lazy_static = "0.2"
+
+[target.'cfg(any(target_os="macos", target_os="windows"))'.dependencies]
+lazy_static = "0.2"

src/lib.rs

@@ -23,7 +23,7 @@ extern crate kernel32;
 extern crate gdi32;
 #[cfg(target_os = "windows")]
 extern crate user32;
-#[cfg(target_os = "windows")]
+#[cfg(any(target_os="macos", target_os="windows"))]
 #[macro_use]
 extern crate lazy_static;
 

src/platform/with_cgl/native_gl_context.rs

@@ -5,9 +5,14 @@ use core_foundation::bundle::{CFBundleGetBundleWithIdentifier, CFBundleGetFuncti
 use core_foundation::base::TCFType;
 use core_foundation::string::CFString;
 use std::str::FromStr;
+use std::sync::Mutex;
 
 use platform::NativeGLContextMethods;
 
+lazy_static! {
+    static ref CHOOSE_PIXEL_FORMAT_MUTEX: Mutex<()> = Mutex::new(());
+}
+
 pub struct NativeGLContextHandle(CGLContextObj);
 
 unsafe impl Send for NativeGLContextHandle {}
@@ -94,6 +99,12 @@ impl NativeGLContextMethods for NativeGLContext {
     }
 
     fn create_shared(with: Option<&Self::Handle>) -> Result<NativeGLContext, &'static str> {
+        // CGLChoosePixelFormat fails if multiple threads try to open a display connection
+        // simultaneously. The following error is returned by CGLChoosePixelFormat: 
+        // kCGLBadConnection - Invalid connection to Core Graphics.
+        // We use a static mutex guard to fix this issue
+        let _guard = CHOOSE_PIXEL_FORMAT_MUTEX.lock().unwrap();
+
         let mut attributes = [
             0
         ];