upgrade to Go 1.19
fix janky dragging, mostly on Windows
readme: update list of args; go run command
avoid going off screen
add twitter handle
random walking

Cloned and flipped the assets manually so that I don't have to implement
in-engine flipping logic.
update deps
use vanity domain
sharper icons: 256 on Windows, 512 on macOS
remove redundant runner.os check; add Apple guide
github actions: separate build for each OS

It's dumber but also clearer. I'm sick of debugging dumb GH Action
github actions: proper mac application bundle
longer feeding animation
add syso file to set icon for the .exe

Steps on linux:

    convert icon.png icon.ico
    go install github.com/akavel/rsrc@latest
    ~/go/bin/rsrc -ico icon.ico -o shark.syso
    rm icon.ico
hungry & feeding animation; also app icon

Since the new feeding anim extends 12px upwards, other sprites needed to
be extended as well.

It may have been more proper to implement some sort of per-sprite coords
system but then I decided to just go with the simplest alternative.

Also added app icon and configurable x,y flags.
add tag helper script; block-quote GH release note
reuse git tag msg for GH Release message
Makefile: add ldflags for Windows
readme: mention GH Artifacts requiring login

Also better (png) screenshot.
simplify RightClick logic

Make sure it doesn't interrupt other animations (or itself).