enc_neon: convert FTransformWHT to intrinsics

slightly faster than the inline asm
in practice not much faster than the C-code in a full NEON build, but
still better overall in an Android-like one that only enables NEON for
certain files.

Change-Id: I69534016186064fd92476d5eabc0f53462d53146
 ~30% encoding speedup: use NEON for QuantizeBlock()

also revamped the signature to avoid having to pass the 'first' parameter

Change-Id: Ief9af1747dcfb5db0700b595d0073cebd57542a5
MIPS: MIPS32r1: Added optimization for FTransform

Change-Id: I9384dac483e8f98bcfdd277a0a3d6ec7c7a7b297
Merge "MIPS: MIPS32r1: Added optimization for FTransform"
MIPS: MIPS32r1: Add optimization for GetResidualCost

+ reorganize the cost-evaluation code by moving some functions
to cost.h/cost.c and exposing VP8Residual

Change-Id: Id976299b5d4484e65da8bed31b3d2eb9cb4c1f7d