Fix some surface refcounting issues in display

[ankith26]

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In get_surface, a strong reference was being incref-ed before being returned (double incref issue)

In set_mode there was a decref followed by an incref, which is technically wrong (I suppose in practice it doesn't create an issue, but I fixed it anyways)

[MyreMylar]

In set_mode there was a decref followed by an incref, which is technically wrong (I suppose in practice it doesn't create an issue, but I fixed it anyways)

What about the return here:

https://github.com/pygame-community/pygame-ce/blob/f5f850ac07a6aab001fb2d687879d4ab434e77cd/src_c/display.c#L1333-L1339

Won't removing the decref cause a leak if it returns NULL?