reference: avoid weighting pixels when plane is unweighted

Just because the luma plane is weighted does not mean either of the chroma
planes are also weighted.  If the weight parameters for a given plane are not
present, then just directly use the un-weighted reference plane.
build: update README to not be so specific about yasm 1.2.0
asm: luma_vps[4x4] in avx2: improve 337c->219c
api: change default AQ mode to 1

We've received a lot of feedback that AQ mode 2 is often problematic, but AQ
mode 1 is generally safe and useful.
pixel: nits
entropy: add methods to estimate CU mode decision costs
cmake: allow position independent code to be generally configurable (fixes #91)

Allow the builder to over-ride the default
analysis: typo
cmake: eoln and white-space fixes, slight refactor