Revised NEON implementations of blend.

Reimplement blend intrinsic using wider memory accesses and a few more 8-bit
operations where possible.  Implementations in AArch32 and AArch64 NEON.

Change-Id: I5e56010376b1db1628a911cf09d97baf5af289b3
Optimise YuvToRGB using 16-bit arithmetic.

Reimplement YuvToRGB intrinsic using 16-bit SIMD arithmetic to increase
throughput.  Implementations in AArch32 and AArch64 NEON.

Change-Id: Idd43e383f5147c33b0b546fa822c970de432c19d
Fix bug in blur when processing odd widths

bug 11942856

Change-Id: I10537cb623cf230636f191fd2b84b050d0f42700
Optimisations to blur intrinsic.

Try to keep all data in-register whereever possible, and use only a minimal
circular buffer on the stack when necessary.  Implementations in AArch32 and
AArch64 NEON.

Change-Id: If3dd4932a94ee1cadde46e298b8f6bf14b6c2bdc