draw: reset extra_vp_outputs.slot to zero in widepoint_flush()

draw: reset extra_vp_outputs.slot to zero in widepoint_flush()

Fixes a crash when clearing the window with a quad after drawing large
points.  We were asking the draw module how many vertex shader outputs
there were and got 3 instead of 2.  This led to creating vertices with
too many attributes and trying to read invalid memory.

We reset extra_vp_outputs.slot to zero in the aaline/aapoint stage's
flush functions already.
This omission was just an oversight in the wide_point stage.