bgm_pixelflut: select is bad.

[vifino]

Steps to ensure pixelflut world domination.

Performance:

• taskpool thread pinning
• keep a good fdset for select
• add epoll variant
• add kqueue variant

Features:

• alpha blending

Bugs:

• Doesn't properly exit. Causes CI failure. Mutex/exit FD related?

right now, the pixelflut server works well for big packets on a single client.
but everything else is really limiting concurrency because of that select.

should fix #49.