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.
Merge pull request #60 from L-Henke/directImageScale

Improved image scaling