Revert "Add IOSurface support"

[jdm]

This reverts commit 92f0e69.

[jdm]

@zakorgy Sorry, I need to back these changes out temporarily. We're trying to merge #131 on a short deadline, and the changes required by servo/servo#23509 are making it very challenging. I think we need to add more abstractions that hide the per-platform differences; I made some changes in https://github.com/jdm/rust-offscreen-rendering-context/tree/multiple2 that are based on these changes that introduce a TextureBacking type which I think could make this easier. I will provide more specific pointers for how I think we should modify servo/servo#23509 to make these changes easier to integrate in that PR.

[zakorgy]

@jdm Thanks for notifying me. Please let me know if #131 and the TextureBacking changes are merged and additional works needs to be done on the IOSurface part. Also looking forward for your pointers for servo/servo#23509 .