Don't create new surface/texture when copying Sprite
Remove setAlpha() and setColorKey()

These functions were being used to no effect. We set the alpha for blank
surface using fillImageWithColor(). The only place we used setColorKey()
was on a surface created with createSurface(), which sets the color key
by itself.
Make sure m_ttf_renderable's surface is set to NULL

This prevents a double-free case.
Merge pull request #926 from dorkster/fixes

SDLRenderDevice fixes