Restrict the condition under which paint cursors read use the cursor
location from the the operating-system.
This caused a glitch when dragging UI elements in painting context
popup. Since the paint cursor would display using mouse motion
which was clamped to the window center - an internal detail of hidden
cursor grabbing.
Now only read the cursor coordinates when clamped to a region which
is used for the transform cursor to stay visible even when the cursor
wraps around.
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