point rendering not compatible with custom viewBox about pointr

[mslegrand]

currently we assume everything is in pixels. At the minimum we should support a svg viewBox=c(0,0,1,1) for a completely coordinate free representation. But the point layer has the point radius and coordinates in pixels. We need to be able to specify that the point matrix is in relative coodinates and adjust the rendering of the points accordingly. For example, as a special case, if viewBox is c(0,0,sWH), then the xy coord returned by ptr should be processed to become sWH, and the radius R of the point icon should be adjusted to become min(s*R).

In general, given a viewBox= c(s,t,u,v), a point p=c(x,y) returned from the ptr javascript, needs to be transformed into viewBox coords by viewBox(p)= (p/WH)c(u,v)+c(s,t) . The radius R to be transformed to become min( Rc(u,v)/WH).

While pt tranform is possible via pt preproc, currently there is no way to adjust the radius R of the point icons.