Move framebuffer allocation table allocation, plus code style fixes

Move the framebuffer allocation table initialization from set_caps
to the device_open function (which is called from the start
function).

Properly free the table in the stop function.

Fix a few small indentation anomalities.

Update the README.
Implement preliminary hardware overlay support

Add preliminary hardware overlay support to GstFramebufferSink.
Derived classes can override the get_supported_overlay_types,
prepare_overlay and show_overlay functions.
Update; revamp get_caps, improved scaler interface, added options

The caps negotiation via get_caps was rewritten to make more sense
and with better results.

The hardware overlay support in libgstframebuffer was improved and
tested.

A few properties were added, including preserve_par.
Implement RGB hardware scaling in sunxifbsink

sunxifbsink now implements hardware RGB overlay scaling. Both buffer-pool
and non-buffer pool modes work as provided by libframebuffersink. The
hardware ioctl code was adapted from xf86-video-sunxifb.

I420 YUV mode is not yet supported.
Update README