Svg rasterize resolution fix

i made a patch which would allow the svg backend to make rasterized plots according to the dpi given in savefig. I wanted this in order to have high-res scatter plots. As you probably know it is hardcoded to 72 dpi. The idea how to, came from the pdf backend. I mostly copy pasted some code.

I also added a test for it. Some other tests now fail the image comparison. It is nothing serious apparently. Probably roundoff errors. Except interp_nearest_vs_none. The result of which looks now exactly like the pdf which it did not before.

Also, two other minor changes which i needed

- Compare_images failed for older numy versions. Simple fix ...
- A numpy.float32 variable got passed to the pdfRepr function in
backend_pdf.py. I changed this function to accept this type, too.