Abstract Backend class for screen backends with double buffering support

[piranna]

Thanks for contributing!

• Have you updated CHANGELOG.md?

[piranna]

Can this be reviewed and merged? It only abstracts the creation of screen-based drawing backends allowing to be as basis to draw directly on Linux framebuffer of X11 o Windows GDI or macOS Cocoa, there's no conflicts at all.

Only thing that maybe would have some concerns are the dynamically created objects at https://github.com/Automattic/node-canvas/pull/1540/files#diff-5c8e889223e6698e689baa171f92bcd9R38-R40, but since they are running in their own short-lived thread they will not create a memory leak. Can somebody confirm it, of tell me what could be a better aproach to explicitly destroy them?

[piranna]

Code sync'ed with latest master branch and all tests are fixed and passing in all platforms. Can this pull-request be reviewed and merged? :-)

[piranna]

Can this pull-request be reviewed and merged? :-)