Fix regression changing 3ds screen size

Merge pull request jtothebell#50 from jtothebell/TransparencyFix

Fix sprite transparency for poked values > 1

Merge pull request jtothebell#44 from jtothebell/ExtCmd

Ext cmd

Merge pull request jtothebell#39 from jtothebell/Stretch

Add screen stretching to sdl2 platforms

Merge pull request jtothebell#34 from jtothebell/AltPalette

Add initial support for secondary palette

Add tests around overdraw bug

Merge pull request jtothebell#26 from jtothebell/z8lua-squashed

Replace lua 5.3 with z8lua, add oval and tline support, plus bug fixes

Update readme to reflect memory compatibility

Merge pull request jtothebell#12 from jtothebell/pngCart

Add support for Png Carts

Update Readme and License