That way, the offscreen can create images of different types.
Its not used in this commit, but will come in handy when we want to
support high bit depth.
This is unused now that all the code uses map/unmap.
The only thing that map/unmap doesn't do that the old code did, was use
a staging image instead as alternative to a staging buffer for image
uploads.
However, that code is not necessary for anything, so I'm sure we can do
without.
As an alternative to gsk_vulkan_image_new_from_data() that
takes a given data and creates an image from it, add a 3 step process:
gsk_vulkan_image_new_for_upload()
gsk_vulkan_image_map_memory()
/* put data into memory */
gsk_vulkan_image_unmap_memory()
The benefit of this approach is that it potentially avoids a copy;
instead of creating a buffer to pass and writing the data into it before
then memcpy()ing it into the image, the data can be written straight
into image memory.
So far, only the staging buffer upload is implemented.
There are also no users, those come in the next commit(s).
