If an i4x4 dct block has no coefficients, don't bother with dequant/zigzag/idct.  Not useful for larger sizes because the odds of an empty block are much lower.
Cosmetics in i16x16 to be more consistent with other similar functions.
Add an SSD threshold for chroma in probe_skip to improve speed and minimize time spent on chroma skip analysis.
Rename lambda arrays to lambda_tab for consistency.