Improved image scaling

Images are now scaled directly at creation. This allows to load even
large images without out of memory problems.

It further allows to set the max scaling factor according to device
limitations.

Since the inSampleSize scaler can only scale to factors of 2, the
resulting image size is somewhere between MAX_PIXEL/2 and MAX_PIXEL.

This could fix the out of memory errors forever. We might add a option where the user can set the max image size to small, medium or large (if the current value doesn't fit all devices).

I could load 20Mpix images with this change which otherwise directly caused an out of memory error.

I would like to have this change in the current alpha release!