Fix error type for unavailable pixel format error

All backends reported this as GLFW_PLATFORM_ERROR, while the
documentation mandates GLFW_FORMAT_UNAVAILABLE.

src/wgl_context.c

@@ -250,7 +250,7 @@ static GLboolean choosePixelFormat(_GLFWwindow* window,
     closest = _glfwChooseFBConfig(desired, usableConfigs, usableCount);
     if (!closest)
     {
-        _glfwInputError(GLFW_PLATFORM_ERROR,
+        _glfwInputError(GLFW_FORMAT_UNAVAILABLE,
                         "WGL: Failed to find a suitable pixel format");
 
         free(usableConfigs);