Change-Id: Ieebea089095d9073b3a94932791099f614ce120c

The output of simple_decoder, as well as the default format for the
ivf{enc,dec} tools, is I420. The encoder examples are changed to default
to I420 as well. This is desirable because passing the output of
simple_encoder to simple_decoder would otherwise have the chroma planes
flipped, which is unexpected.

Change-Id: I9d4f10ace594a54a7fc4f9a1f9187365c01fd638

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