SDL: Avoid having SDL_SRCALPHA set even if we have alpha in the pixelformat.

There are issues with WME producing "ghosting"-artifacts in Dirty Split and Chivalry when using the SDL-rendering backend. These do not show when running the OpenGL-backend.

Looking into it a bit, it seems that explicitly disabling SDL_SRCALPHA on the _screen surface solves this issue. (As the documentation states that this is implicitly enabled when non-0 alpha-masks are supplied).