Only change the multiplication factor and not the original size when using area scaling method

Currently, the area scaling method for point symbols is not very satisfactory, as it is simply taking the square root of the final size of the symbol, and this individually for each layer. 

Issues:
* The area is not the multiple of the original area as expected. For example, some elements are smaller than the original size.
* The different layers of the symbol are scaled differently, based on their individual size, which changes the aspect of the symbol (here I have three circles of size 10, 6 and 2, so the green band, red band and black circle should be of the same size)

With this pull request, only the multiplying factor is modified with a square root, which means a size scale field of 1 is the original symbol, 2 for twice the area, 4 for twice the diameter, etc.

For an original symbol such as: 
![original symbol](https://cloud.githubusercontent.com/assets/537624/3073216/fdb1cafc-e2de-11e3-89cb-0e32e1662097.png)
we previously had something like:
![previous area scaling](https://cloud.githubusercontent.com/assets/537624/3073217/0c1dc4d8-e2df-11e3-929e-42184fc69a0d.png)
With the pull request, the end result is: 
![new area scaling](https://cloud.githubusercontent.com/assets/537624/3073334/ec8b80a8-e2e6-11e3-8836-5072e9847465.png)