Fixed: Improve handling of empty ';' SGR sequences

Currently the Termux terminal emulator prints "HI" in red with:

```sh
printf "\e[31;m HI \e[0m"
```

This is not how other terminals (tested on xterm, gnome-terminal,
alacritty and the mac built in terminal) handle it, since they parse
""\e[31;m" as "\e[31;0m", where the "0" resets the colors.

This change aligns with other terminals, as well as improves performance
by avoiding allocating a new int[] array for each byte processed by
`parseArg()`, and most importantly simplifies things by removing the
`mIsCSIStart` and `mLastCSIArg` state, preparing for supporting ':'
separated sub parameters such as used in
https://sw.kovidgoyal.net/kitty/underlines/