Improve .gitignore for doc and submit_awcy.py
Add SSE4.1 and AVX2 detection.

We'll want to renumber the flags they when we add other instruction
 sets, but this doesn't affect ABI, so that can be done later.

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>
Add SSE2/SSE4.1 version of od_bin_fdct4x4.

Currently requires configuring with -msse4.1 in your CFLAGS to build
 the SSE4.1 version.

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>
Add run-time CPU detection for the 4x4 DCT.

Also tests all implementations in dcttest.
This is not actually the test we want to run: we want to test that
 they match the C version exactly, using OD_CHECKASM similar to how
 sse2mc.c does it.
However, it's a start.

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>
Add SSE2/SSE4.1 version of od_bin_idct4x4.

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>