rustc is now making use of the fact that derefencing a null pointer is UB. Remove all null dereferences

[rukai]

• Added an entry to CHANGELOG_VULKANO.md or CHANGELOG_VK_SYS.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes - in this repository
• Updated documentation to reflect any user-facing changes - PR to the guide that fixes existing documentation invalidated by this PR.

Uh oh..
rustc is now making use of the fact that derefencing a null pointer is UB.
So we need to remove all null dereferences, of which there are surprisingly large amount of. In order to get vulkano working AT ALL on nightly and presumably soon beta + stable.

@tomaka You have a pretty good idea what is and isnt UB. Why did you write these null dereferences, is there something you needed from them that I am missing?

Fixes #1207

[rukai]

Ok, this PR now runs on the latest nightly.

Unfortunately I could not find a way to do so without a breaking change.
The vertex struct provided by the user now needs to #[derive(Default)]
If anyone can provide a fix that isn't breaking I will gladly take it!

I'll wait a few days to get feedback from people. @mitchmindtree @freesig
Then I'll go ahead and release this as a breaking change so people can transition before vulkano breaks on stable rust.

[tomaka]

It was a way to be sure that the offset is actually correct. In other words, a replacement for the offsetof! macro that Rust is lacking.

[rukai]

Is the variable called size poorly named and should really be another type of offset?

I just replaced the size stuff with size_of which worked on everything I tested, but might still be wrong?

[tomaka]

I guess size_of should work, but is not necessarily 100% future-proof. In practice it should be fine, I guess.