Don't change resolve mask when setting brush doesn't change palette

After 556511f9f39ddc887481e0cd5a877134ceb0da6b, which moved the resolve
mask storage into the palette's d-pointer, modifying the resolve mask
requires a detach. As of now, we only detached when setting a different
brush, but always modified the resolve mask, which broke palettes that
shared the d-pointer (likely the global default palette).

However, detaching has negative side effects when styles set brushes on
temporary palette objects and then use that palette object's cache key
to build a cache of pixmaps. As each drawing would detach the palette
(even if the palette doesn't change, which is likely), the cache key
changes with each detach, and the cache would quickly increase in size.
This was addressed in changes d7bcdc3a442b99c2caebd4cfd38de67e14090e05
and 1e75dcf2518e37a7b83d006a4e002f972615b43b.

We can either detach and find other ways to address the issues from
QTBUG-65475, or we can not change the resolve mask when the brush doesn't
change and completely ignore the call.

Since QFont ignores the setting of any attribute to a value that is
identical to the current value, and since it's possible to force that
the resolve-bit is set by calling setBrush twice with different brushes,
ignoring the call seems like the better solution.

[ChangeLog][QtGui][QPalette] Setting a brush on a palette that is
identical to the current brush no longer sets the resolve mask bit for
that particular role, so items using the palette will continue to
inherit changes from parent items.

Fixes: QTBUG-98762
Task-number: QTBUG-65475
Pick-to: 6.2
Change-Id: Ife0f934b6a066858408ef75b7bb7ab61193ceb47
Reviewed-by: Simon Hausmann <hausmann@gmail.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

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