[PointerEvent algorithms] MouseEvents are created from PointerEvents

[mustaqahmed]

PointerEvent attributes are populated from low-level (system-level) events. UIEvents Section 5.3.3. create PointerEvent from MouseEvent assumes they are populated from a MouseEvent which is not the case. The set of attributes in a MouseEvent is a subset of those in a PointerEvent.

Possible solution

We can move the section to the PointerEvent spec after reversing the construction direction.

Is it acceptable from spec perspective to omit the conversion as implied? The important thing to retain is that UIEvents should own the definition and initialization of MouseEvent attributes. But we can handle that trivially and have the PE spec own when that init happens.

[mustaqahmed]

FYI: @garykac @flackr @smaug----