EGL: Add support for EGL_EXT_platform_base

This adds support for EGL_EXT_platform_base and its associated X11 and
Wayland extensions, allowing us to explicitly tell EGL which window
system we are using.

This is based on work by @linkmauve in #1691.

Closes #1691.

src/cocoa_window.m

@@ -1721,6 +1721,11 @@ const char* _glfwPlatformGetClipboardString(void)
     } // autoreleasepool
 }
 
+EGLenum _glfwPlatformGetEGLPlatform(void)
+{
+    return 0;
+}
+
 EGLNativeDisplayType _glfwPlatformGetEGLNativeDisplay(void)
 {
     return EGL_DEFAULT_DISPLAY;