Fix wlfreerdp pointer cursors

[jadahl]

Two commits:

uwac/input: Set the right serial when setting cursor

The serial in wl_pointer.set_cursor must exactly match the one from
wl_pointer.enter, it should not use whatever serial for any input class

and

uwac/input: Don't pass hotspot as offset when attaching cursor buffer

Passing a non-zero offset to cursor buffer and then calling
wl_pointer.set_cursor doesn't make much sense, as any offset will
immediately be reset. The protocol specifies the cursor set by
wl_pointer.set_cursor to be

> The parameters hotspot_x and hotspot_y define the position of
> the pointer surface relative to the pointer location. Its
> top-left corner is always at (x, y) - (hotspot_x, hotspot_y),
> where (x, y) are the coordinates of the pointer location, in
> surface-local coordinates.

This leaves no room available for any previously set offsets to be kept

[freerdp-bot]

Can one of the admins verify this patch?

[akallabeth]

@freerdp-bot please test this nice pr!

[freerdp-bot]

Refer to this link for build results (access rights to CI server needed):
https://ci.freerdp.com//job/PullRequestTester/8393/

[hardening]

LGTM