Add vendored cairo-sys-rs crate

[wez]

I'm experimenting with using cairo's 2D graphics for glyph related drawing operations, and perhaps more stuff in the UI in the future.

The challenge is that no one has yet encapsulated its build in a cargo-friendly way.

This commit is that attempt.

The strategy is this:

• deps/cairo is ostensibly the same thing as https://github.com/gtk-rs/gtk-rs-core/tree/master/cairo/sys which is the cairo-sys-rs crate
• Its build.rs has been replaced with a version that builds from the C sources that are found in-tree; it will never look for the system cairo library
• A pair of scripts to import pixman (a required dep of cairo) and cairo itself in a repeatable way are include, for if/when we need to update the vendored sources Cairo is a mature library and doesn't release very often.
• The workspace sets a patch for crates.io so that cairo-rs has its cairo-sys-rs dep redirected to the local build.

I'm not planning to upstream this stuff to gtk-rs-core as full cairo depends on a lot more things than are dealt with here, so it would be a PITA to handle that generally.

This vendoring strips out things that are not used, so the cairo library that remains:

• Is not internally thread safe; no TLS or mutex support is enabled in either of them. That's fine because the high level cairo-rs wrapper is not Send+Sync, which means that rust will only allow it to be used single threaded anyway.
• Only has the basic recording and image surfaces
• No platform support for win32/xlib/xcb and so on