fbdev_window: cleanups

I've used the same idea from wayland_window.cpp, having
the buffers placed in a std::list and marked busy/free via the busy
member.

Fixed the crash which apparently was caused by reference counting but
in fact was dequeueBuffer not calling incRef on the buffer.

After debugging with Carsten, fb->post is now used correctly by
adding checks for frontbuffer in ::dequeueBuffer.

[carsten] nailed the second cond_wait when cond was signaled but no
buffers were actually available

v3: remove custom TRACE

v2: changed licensing from lgpl to apache2
    use DEBUG which is enabled bt --enable-debug

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>