Push: Don't convert pixel.RGBA to pixel.RGBA

Because that's expensive, even in the case where the conversion
is trivial. Use type assertion first. Reduces runtime cost of
imdraw.Push from ~15.3% to 8.4%, so not-quite-50% of runtime
cost of pushing points.

If you were setting imd.Color to Color objects that aren't RGBA
every single point, not much help. But if you set it and then
draw a bunch of points, this will be a big win.