Disallow skipping transform and quantization

Disallow skipping transform and quantization

The encoder has a feature to skip transform and quantization based
on model rd analysis. It could happen that the model
based analysis lets the encoder skips transform and quantization, while
a bad prediction occurs, leading to bad reconstructed blocks, which
are intrusive and apparently coding errors.

We add a speed feature to guard the skipping feature.
Due to the risk of bad perceptual quality, we disallow such skipping
by default.

On hdres test set, speed 2, the coding performance difference is 0.025%,
speed difference is 1.2%, which can be considered non significant.

BUG=webm:1729

Change-Id: I48af01ae8dcc7a76c05c695f3f3e68b866c89574