add int64 type conversion to sourdump encode and decode

[gitmpr]

fixes the sourdump go build for me using go version go1.23.2 linux/amd64 which might be more strict on requiring explicit int64 types than go version go1.23.3 linux/amd64 that is used in CI to build sourdump

/sour/assets](main)$ bash -x setup
+++ dirname setup
++ cd .
++ pwd
+ SCRIPT_DIR=/home/mpr/git/sourfork/sour/assets
+ cd /home/mpr/git/sourfork/sour/assets
+ set -e
+ mkdir -p cache
+ pip3 list
+ grep cbor2
+ '[' -f sourdump ']'
+ cd ..
+ go build -o assets/sourdump cmd/sourdump/main.go
# github.com/cfoust/sour/pkg/maps
pkg/maps/decode.go:22:47: cannot use i (variable of type int) as int64 value in argument to worldio.CubeArray_getitem
pkg/maps/decode.go:31:68: cannot use j (variable of type int) as int64 value in argument to worldio.SurfaceInfoArray_getitem
pkg/maps/decode.go:41:59: cannot use j (variable of type int) as int64 value in argument to worldio.UcharArray_getitem
pkg/maps/decode.go:47:62: cannot use j (variable of type int) as int64 value in argument to worldio.Uint16Array_getitem
pkg/maps/encode.go:57:37: cannot use i (variable of type int) as int64 value in argument to worldio.CubeArray_setitem```