Fixes to halide_image_io.h
Fixes to halide_image_io.h

-- JPEG case needed to use convert(), otherwise saving  non-uint8 images as JPEG produced garbage
-- improved rounding of the convert() functions (e.g. mul or div by 257 instead of 256); this may be overthinking it, but the extra overhead is unlikely to be meaningful here
-- added convert functions for bool and int64 types; these aren't likely to be very useful in practice, but it means we can instantiate compilable code for all of the supported Halide::Type C++ equivalents, and doesn't really cost us anything

kgnk/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide,psuriana/Halide,psuriana/Halide,kgnk/Halide,psuriana/Halide,psuriana/Halide,psuriana/Halide