Early termination for chroma encoding
Faster chroma encoding by terminating early if heuristics indicate that the block will be DC-only. This works because the vast majority of inter chroma blocks have no coefficients at all, and those that do are almost always DC-only. Add two new helper DSP functions for this: dct_dc_8x8 and var2_8x8. mmx/sse2/ssse3 versions of each. Early termination is disabled at very low QPs due to it not being useful there. Performance increase is ~1-2% without trellis, up to 5-6% with trellis=2. Increase is greater with lower bitrates.
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