[webgl-point] Misc changes

This PR adds three commits:

- 9029c0f changes the coordinate system origin from the replay maxExtent bottom left corner to the maxExtent center. Using the center makes more sense, are, on average, points are closer to the center than the bottom left corner.

- 64a7cdf adds `getImageSize` to `Circle` and `RegularShape`. @tsauerwein, `getSize` returns the size of the sub-rectangle in the original size, while `getImageSize` returns the actual size of the image. Currently, for `Circle` and `RegularShape`, `getImageSize` and `getSize` return the same value, but this will change when symbols work with atlases.

-  43756a2 adds support for the image style `scale` option.