windows support

[johnnychen94]

Unfortunately, the cross-compiled libsixel dll library doesn't provide one key symbol that we need: sixel_allocator_new and thus our current implementation doesn't work on Windows. A possible workaround (not yet checked) is to create our Julia version of SixelAllocator and then pass it to the C side libsixel.

@saitoha Is this an expected behavior or I did something wrong with the libsixel compilation? The build script can be found in https://github.com/JuliaPackaging/Yggdrasil/blob/master/L/libsixel/build_tarballs.jl (cross-compiled from mingw)

all exported symbols in windows dll

[johnnychen94]

This turns out to be a missing SIXELAPI to sixel_allocator_new; PRs opened in both upstream libsixel(saitoha/libsixel#151) and Julia Yggdrasil(JuliaPackaging/Yggdrasil#2964) as hot-fix.