Add check for EGL specific buffer swap issue

Fixes #675.

README.md

@@ -96,6 +96,8 @@ used by the tests and examples and are not required to build the library.
  - [GLX] Bugfix: NetBSD does not provide `libGL.so.1`
  - [EGL] Added `_GLFW_USE_EGLPLATFORM_H` configuration macro for controlling
          whether to use an existing `EGL/eglplatform.h` header
+ - [EGL] Added and documented test for if the context is current on the calling
+         thread during buffer swap
  - [EGL] Removed dependency on external EGL headers
 
 

include/GLFW/glfw3.h

@@ -3429,6 +3429,9 @@ GLFWAPI GLFWwindow* glfwGetCurrentContext(void);
  *
  *  @param[in] window The window whose buffers to swap.
  *
+ *  @remarks __EGL:__ The context of the specified window must be current on the
+ *  calling thread.
+ *
  *  @par Thread Safety
  *  This function may be called from any thread.
  *

src/egl_context.c

@@ -584,6 +584,13 @@ void _glfwPlatformMakeContextCurrent(_GLFWwindow* window)
 
 void _glfwPlatformSwapBuffers(_GLFWwindow* window)
 {
+    if (window != _glfwPlatformGetCurrentContext())
+    {
+        _glfwInputError(GLFW_PLATFORM_ERROR,
+                        "EGL: The context must be current on the calling thread when swapping buffers");
+        return;
+    }
+
     eglSwapBuffers(_glfw.egl.display, window->context.egl.surface);
 }