Add special_flags to Group.draw()

[MarcellPerger1]

Add special_flags to Group.draw(), fixes #3306.
I implemented this in the same way as in pygame/pygame#3722 for compatibility with pygame/pygame:

• I added bgsurf as a (sometimes unused) 2nd argument to to all the Group.draw() methods so that special_flags is consistently the 3rd argument across all subclasses.
• For LayeredUpdates.draw(), special_flags (if provided as non-None) overrides the blendmode of sprites.

In the docs, I haven't added the versionchanged because I don't know if the next version (or version where this will be merged) will be 2.5.3 or 2.6.0.

I haven't used fblits in this PR as I thought it might be better to do that in separate PR (let me know if I should do it in this PR)

[Starbuck5]

In the docs, I haven't added the versionchanged because I don't know if the next version (or version where this will be merged) will be 2.5.3 or 2.6.0.

2.5.4, as of today :)

[aatle]

special_flags should default to None.

[aatle]

The feature is nice, but the implementation looks strange (it can't be changed though, for pygame compatibility).
Rather than adding a confusing unused bgsurf argument for all sprite groups to keep consistency with a single, rarely used LayeredDirty subclass, it would have been better to make special_flags keyword-only. Every code example given has used it as a keyword argument.
I also don't see why has bgd been renamed to bgsurf. This is technically a breaking change for LayeredDirty (though it probably won't have much impact).

[MarcellPerger1]

I also don't see why has bgd been renamed to bgsurf. This is technically a breaking change for LayeredDirty (though it probably won't have much impact).

The only reason I changed it here is for compatibility with pygame, although it might be better just to keep it as bgd for compatibility with previous versions of pygame-ce (personally, I think compatibility with previous versions of pygame-ce is more important than compatibility with pygame).
I can change it back to bgd if you think that would be better.

[Starbuck5]

(personally, I think compatibility with previous versions of pygame-ce is more important than compatibility with pygame).

I agree.