oxr: check input type in oxr_swapchain_gl_enumerate_images

Don't blindly cast the input struct to XrSwapchainImageOpenGLKHR, check that the type is XR_TYPE_SWAPCHAIN_IMAGE_OPENGL_KHR.

I'm not sure we should use OXR_GET_INPUT_FROM_CHAIN here. If not, should I just replace it with a type check and a cast?