Merge STYLE_TOPDOWN and STYLE_TOPDOWN_TIGHT

There was a lot of repeated code.
Rename variables in `FlxCamera#follow()` to be more descriptive
Only set `deadzone` if the camera is set to follow a target
Remove "platformer offset" from `LOCKON` style
Add error message if invalid follow style is used

Also, fix tiny typo from a previous commit.
Merge branch 'refactor-and-fix-flxcamera' of https://github.com/IQAndreas/flixel