endian_inl.h: add BSwap16

+ use it in VP8LoadNewBytes()

Change-Id: I701d3652dc0cbd553852978702ef68c2657bca1c
endian_inl.h: implement htoleXX with BSwapXX

+ s/htole(16|32)/HToLE$1/ to avoid any name conflicts

Change-Id: Ic1c84711557e50f73d83ca5aa2b3992ac6738216
VP8LFillBitWindow: respect WEBP_FORCE_ALIGNED

Change-Id: I23eddf01590de002efc21d8c7acc545a08fc3e48
VP8LFillBitWindow: enable fast path for 32-bit builds

also reduce the load size from 64 to 32 bits as the top 32 bits are
being shifted away in the operation.

the change is neutral speed-wise on x86_64 as is the change in load size
on x86, but it gives a slight improvement on 32-bit arm.
x86 is improved ~13%, 32-bit arm ~3.7%
aarch64 is untested but will likely benefit as well.

Change-Id: Ibcb02a70f46f2651105d7ab571afe352673bef48