Remove the last remains of slave mode

Almost nothing was left of it.

The only thing this commit actually removes is support for reading
input commands from stdin. But you can emulate this via:

 --input-file=/dev/stdin --input-terminal=no

However, this won't work on Windows. Just use a named pipe.
win32: fix assertion failure

If OpenGL 3.x doesn't work, the fallback to legacy GL will call the
function to create the DC again, and it will assert. Just make it not
to, and also simplify the code a bit.

Fixes #974.