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.