This patch creates some basic infrastructure for doing bitstream-
incompatible changes to the VP8 encoder. The key parts are:

 - --enable-experimental configure switch, to enable support for this
   incompatible bitstream. This switch is required to be set to enable
   any "experiments"

 - A list for "experiments" which translate into --enable-<experiment>
   options and CONFIG_<experiment> macros.

 - The high bit of the "Version" field is used to indicate that the
   bitstream was produced by an experimental encoder. The decoder will
   fail to decode an experimental bitstream without
   --enable-experimental.

 - A new "vp8x" encoder interface is created to set the experimental
   bit.

 - The vp8x encoder interface is made the default for ivfenc in
   experimental mode.

Change-Id: Idbdd5eae4cec5becf75bb4770837dcd256b2abef

Change-ID: I093abe6094589a0d73f6ca85b825678a19e68285

The PLANE_{PACKED,Y,U,V,ALPHA} macros should be renamed to be within the
VPX_ namespace.

Fixes #27

This renames the vpx_codec/ directory to vpx/, to allow applications
to more consistently reference these includes with the vpx/ prefix.
This allows the includes to be installed in /usr/local/include/vpx
rather than polluting the system includes directory with an
excessive number of includes.

Change-Id: I7b0652a20543d93f38f421c60b0bbccde4d61b4f

Avoid an potential name clashes and match other external types.
s/IMG_FMT/VPX_$&/g
s/img_fmt/vpx_$&/g

Change-Id: Ia7ad5bbb6424416b37e71e5f5eb1eca31c3c707f

This doesn't play well with autotools, and the preprocessor magic is
confusing and unhelpful in the vp8-only context.

Change-Id: I2fcb57e6eb7876ecb58509da608dc21f26077ff1