Implement a minimal EGL -> CGL backend This succesfully renders HelloTriangle and some samples but fails on a lot of tests. In particular it doesn't handle resizing the window and doesn't have depth or stencil buffers. BUG=angleproject:891 Change-Id: I16356471b470f764acb38e8dd3589e9c0129829d Reviewed-on: https://chromium-review.googlesource.com/290770 Tested-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>