We apply a default window background from the palette to all `Window`
elements, and likewise we need to apply the default text color to Text
elements to ensure a readable contrast.
This also fixes the default text color when using Qt/native style in
dark mode on macOS.
* Change SharedVector::as_mut_slice() to SharedVector::make_mut_slice()
* SharedPixelBuffer::as_mut_slice and SharedPixelBuffer::make_mut_slice()
* SharedPixelBuffer::as_bytes_mut() to SharedPixelBuffer::make_mut_bytes()
This is consistent with Rc::make_mut() and is supposed to emphasize
that, unlike the usual as_* operations, it is not a free operation but
may involve work (detaching the data by cloning it).
Amends commit 7384ebdc32
