47% of the time is spent in BoolReader::read_bool when decoding lossy images

[Shnatsel]

Decoding this file with image-webp is 1.8x slower than dwebp -noasm -nofancy:
Puente_de_Don_Luis_I,_Oporto,_Portugal,_2019-06-02,_DD_29-31_HDR.webp.gz

(source: wikipedia, converted to lossy WebP using imagemagick)

Profiling with samply shows image_webp::vp8::BoolReader::read_bool being responsible for 47% of the total runtime when decoding this image: https://share.firefox.dev/49ZwlOo

This seems to be a similar issue to #55

[fintelia]

Equivalent function in libwebp

Unfortunately, unlike with Huffman compression I don't think it is possible to avoid the bit-by-bit decoding for arithmetic coding.

[Shnatsel]

Unfortunately #72 didn't seem to make a difference on benchmarks on my machine. Decoding the linked image is still 1.8x slower than dwebp -noasm -nofancy.

I've grepped libwebp source code for VP8GetBit and I couldn't find any specialized assembly routines for it, but they do have a dedicated variant for prob=0x80 (128 in decimal), so that's something we could probably crib for easy gains. prob=128 is used here and here.

There is also a VP8GetBitAlt function the purpose of which isn't clear to me at a glance.