doc: Add changelog snippets for !444

doc/changes/auxiliary/mr.444.1.md

+u/vk: Remove unused vk_image struct, this is later recreated for the image
+allocator code.

doc/changes/auxiliary/mr.444.2.md

+u/vk: Add a new image allocate helper, this is used by the main compositor to
+create, export and import swapchain images.

doc/changes/ipc/mr.444.1.md

+generation: Fix handling 'in_handle' by adding a extra sync round-trip, this
+might be solvable by using `SOCK_SEQPACKET`.

doc/changes/ipc/mr.444.2.md

+Implement the `xrt_compositor::import_swapchain` function, uses the earlier
+`in_handle` work.

doc/changes/xrt/mr.444.1.md

+Make some fields on `xrt_gl_swapchain` and `xrt_vk_swapchain` private moving
+them into the client compositor code instead of exposing them.

doc/changes/xrt/mr.444.2.md

+Make `xrt_compositor::create_swapchain` return xrt_result_t instead of the
+swapchain, this makes the methods on `xrt_compositor` more uniform.

doc/changes/xrt/mr.444.3.md

+Add the method `xrt_compositor::import_swapchain` allowing a state tracker to
+create a swapchain from a set of pre-allocated images. Uses the same
+`xrt_swapchain_create_info` as `xrt_compositor::create_swapchain`.