Since it is only available for testing.

Implement a pure-rust pred_dc

And add some tests.

Fix #2: Enable Travis

Add (but do not use) H_PRED support

If chrome uses H_PRED then the transform type is DCT_ADST, but we were
always using DCT_DCT, we now lookup the correct transform type using
`exported_intra_mode_to_tx_type_context`.

Fixing this also requires using the correct scan table for each
transform type. This is handled using the lookup table
`av1_intra_scan_orders`.

The encoder still uses DC_PRED everywhere until we get RDO in.
This required turning off ext-intra to work.

Note that this only works for luma blocks, H_PRED support for chroma
blocks is fixed in a latter commit in this PR.

- Mode::pred -> Mode::mode since we use this in function like
write_intra_mode, etc.
- SubMode::pred -> SubMode::mode

- Mode::new() -> Mode::default() since "new" is a bit vague.
- SubMode::new() -> SubMode::default()

Use correct AV1 fourcc

Avoid weird do-while loop

Rewrite od_ec_enc in Rust

Simplify a little update_cdf

Make ContextWriter a struct of values, not references

Use a git submodule for aom instead of copying files

The submodule is based on https://github.com/tdaede/aom/commits/rav1e