This fixes the rendering of transparent animations when using a 
background thread.

Add tests for these circumstances.

This makes SPI_GETUIEFFECTS return TRUE by default.

Calculating an open-ended structure using sizeof doesn't yield an 
accurate size because of alignment.

comctl32: In certain circumstances, native comctl32's animation control accepts negative start positions for the ACM_PLAY message.

These circumstances seems to be after a seek is performed, but we play
it safe and allow negative start positions to be treated as zero always.