racket/draw: faster (on some platforms) path for drawing a bitmap

The faster path applies in the common case of drawing a whole
bitmap, as opposed to a section of the bitmap.

Thanks to Laurent Orseau for tracking down this performance issue.
games/cards: add @2x card images

For both original and @2x, adjust images to use alphas for the corner,
instead of drawing with clipping, which avoids alignment problems on
an @2x display.
remove/adjust obsolete C preprocessor cases

Mac OS Classic and Palm ae long since unsupported.

TCP support implies sockets (since old Mac TCP is gone).

For what it's worth, make the build work without TCP support, although
no one ever builds that way.
faster path for TCP input

Avoid polling a file descriptor to determine whether it has bytes;
just try to read, and then go to epoll/kevent mode when available,
skipping a poll/select.
reduce calls to `current-process-milliseconds` on thread swap

Call `current-process-milliseconds` once per swap, instead of twice.