Change target_os to windows instead of win32
Add RenderTarget trait.

Updated RenderWindow and RenderTexture to use it,
and appropriate imports where needed.  Should probably
update the Drawable trait now, as that's where the most
benefit will show up.

This is a minorly breaking change as it needs an extra import
and possibly some changes where views for RenderWindows are needed.
Change Drawable to use the new RenderTarget trait.
Merge pull request #49 from brandonson/render-target-trait

RenderTarget trait