Fix segfault in surface.fblits

[Matiiss]

Moved reference decrementing to after the values are used for stuff (see https://docs.python.org/3/c-api/iter.html#c.PyIter_Next).
I'm not sure why this wasn't causing issues for like actual tuples like (0, 0) being used directly for the destination, but it appears this was related to the object being passed having to get an attribute dynamically? Not entirely sure, but the segfault seems to have gone away.

MRE:

import pygame

pygame.init()
screen = pygame.display.set_mode((640, 360))
surf = pygame.Surface((10, 10))
screen.fblits((surf, surf.get_rect().topleft) for _ in range(1))
print("finished")

[Matiiss]

Also with these changes in place it's possible to wrap most of the functionality of both cases (lists/tuples and generators) in a single helper function, thus reducing repetition (call one function in two places), should that be done in this PR or a separate one?

[itzpr3d4t0r]

LGTM! I guess we could do it either way, so it's your choice.

[Matiiss]

Alright then, since this PR focuses on fixing a bug, I will submit another PR focusing on code robustness once this gets merged.

[ankith26]

LGTM, thanks for the PR 🥳