This allows renderers (or anyone really) to attach "render data" to
textures. Only the first render data sticks.

You can gsk_texture_set_render_data() with the key you will use to
look the data up again, and if no data has been set yet, yours will be
set.

You can retrieve this data via gsk_texture_get_render_data() later on.
If your data has been cleared, NULL will be returned.

When gsk_texture_clear_render_data() is called (which the texture will
call when it is finalized), your destory notify will be called and you
have to release your render data.

The GL driver uses this to attach texture ids to GskTextures.