demos/spiv: Fix old bug with image dragging
demos/spiv: Fix old bug with image dragging

If a big image is dragged around by a mouse the process of blitting is
generally slow, after this change the thread that draws image onto
window aborts the operation if there are subsequent mouse pointer
events.

Signed-off-by: Cyril Hrubis <b1e90efd3b808b8e0bcf2a9a55fd8b9a779bfd81@ucw.cz>

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