This function replaces some uses of GetContentScaleFactor(), where a
factor greater than 1 must be used even under the platforms not doing
any logical/physical pixel mapping, such as MSW.
For now GetContentScaleFactor() is still unchanged, but it will return 1
for such platforms in the future and adding GetDPIScaleFactor() allows
to avoid changing the behaviour of the code which relied on its current
behaviour.
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