Replace wlr_XXX_is_YYY and wlr_XXX_from_YYY

[emersion]

We have lots of those.

For instance, we have:

• wlr_dmabuf_resource_is_buffer which checks if a resource is a buffer
• wlr_dmabuf_buffer_from_buffer_resource which asserts that the resource is a buffer and returns it

We could deduplicate and simplify that into just one wlr_dmabuf_buffer_from_buffer_resource function which returns NULL if the resource is not a DMA-BUF. Callers can assert the returned value if they want to. This would be more consistent with the Wayland API which has:

struct wl_shm_buffer *wl_shm_buffer_get(struct wl_resource *resource)

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wlroots has migrated to gitlab.freedesktop.org. This issue has been moved to:

https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/884