ENH: Builder ExpSettings now supports blendMode=add/avg but the FBO isn't working correctly
BF: visual.py should have the openWindows list in its namespace
ENH: Builder timing now stops at t-win.monitorFramePeriod*0.75 rather than t

This should take into account the the final frame of a stimulus presentation
Components using duration(s) or stopTime(s) now check if there's "most of"
one frame remaining and continue to draw if there is.

Why 0.75 frames?:
 - if this is 1 frame then when we go over by a tiny fraction we'll stop drawing
   too early
 - 0.5 frames sounds like it would handle rounding up/down, but doesn't
   account for further drawing time. We could check beam pos, but not
   convinced that's necess
FF: blendMode='add' wasn't scaled correctly - needed adjustment to shaders
FF: in Builder, for blendMode='add' you also need useFBO=True
ENH: plaid demo should use blendMode='add'