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.
7 jobs for enumerate-images-check-type in 4 minutes and 59 seconds (queued for 11 seconds)
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