Add FlxConditionals initalization macro

- adds compile-time errors for swf-version conflicts (previously runtime + dependant on using the flash debug player)
- adds helper conditionals to avoid redundancy
- adds a compiler error when both FLX_RENDER_BLIT and FLX_RENDER_TILE are defined
- makes it so that rendering conditionals can be defined via <haxedef> instead of <set>
- removes some of the conditional-handling logic from the include.xml