core: workaround window not being rendered till moved on macOS Mojave

Apple ought to fix their OpenGL implementation, but with OpenGL now
deprecated this might not happen.

This has been reported upstream in GLFW in glfw/glfw#1334.
The workaround comes from kovidgoyal/kitty@b82e74f99

This also fixes the HiDPI (Retina) scaling issues reported in #497,
so the workaround is enabled anywhere __APPLE__ is defined.

src/core.c

@@ -134,12 +134,6 @@
     #define CHDIR chdir
 #endif
 
-#if defined(__linux__) || defined(PLATFORM_WEB)
-    #include <sys/time.h>           // Required for: timespec, nanosleep(), select() - POSIX
-#elif defined(__APPLE__)
-    #include <unistd.h>             // Required for: usleep()
-#endif
-
 #if defined(PLATFORM_DESKTOP) || defined(PLATFORM_WEB)
     #if defined(PLATFORM_WEB)
         #define GLFW_INCLUDE_ES2
@@ -155,6 +149,15 @@
     #endif
 #endif
 
+#if defined(__linux__) || defined(PLATFORM_WEB)
+    #include <sys/time.h>           // Required for: timespec, nanosleep(), select() - POSIX
+#elif defined(__APPLE__)
+    #include <unistd.h>             // Required for: usleep()
+    #include <objc/message.h>       // Required for: objc_msgsend(), sel_registerName()
+    #define GLFW_EXPOSE_NATIVE_NSGL
+    #include <GLFW/glfw3native.h>   // Required for: glfwGetNSGLContext()
+#endif
+
 #if defined(PLATFORM_ANDROID)
     //#include <android/sensor.h>           // Android sensors functions (accelerometer, gyroscope, light...)
     #include <android/window.h>             // Defines AWINDOW_FLAG_FULLSCREEN and others
@@ -233,6 +236,11 @@ static bool windowReady = false;                // Check if window has been init
 static bool windowMinimized = false;            // Check if window has been minimized
 static const char *windowTitle = NULL;          // Window text title...
 
+#if defined(__APPLE__)
+static int windowNeedsUpdating = 2;             // Times the Cocoa window needs to be updated initially
+#endif
+
+
 #if defined(PLATFORM_ANDROID)
 static struct android_app *androidApp;          // Android activity
 static struct android_poll_source *source;      // Android events polling source
@@ -2870,6 +2878,15 @@ static void SwapBuffers(void)
 {
 #if defined(PLATFORM_DESKTOP) || defined(PLATFORM_WEB)
     glfwSwapBuffers(window);
+#if __APPLE__
+    // workaround for missing/erroneous initial rendering on macOS
+    if (windowNeedsUpdating) {
+        // Desugared version of Objective C: [glfwGetNSGLContext(window) update]
+        ((id (*)(id, SEL))objc_msgSend)(glfwGetNSGLContext(window),
+                                        sel_registerName("update"));
+        windowNeedsUpdating--;
+    }
+#endif
 #endif
 
 #if defined(PLATFORM_ANDROID) || defined(PLATFORM_RPI) || defined(PLATFORM_UWP)