Use this class to draw points and circles which size has to be specified in
screen units. Points (and circles resulting from this class) do not have
area,
however during rendering to make the point visible at varying zoom levels
a buffer area is built around it.
To give the point custom size, create a GeometryBuffer with, set buffer to
your value and assign the point the final style.
Note that since points do not have any area, they are not generalized.
Normally points retain their size in the screen units across all zoom levels
but this can be customized. Use setStartLevel on the point's style to specify
from which zoom level the point should "stick to the map" and not decrease in
size.