use aspect-preserving bitmap scalers with MMS images

This fixes issue #246 where portrait photos are incorrectly resized/stretched when used with MMS messages.

These new aspect-preserving routines are used when creating a thumbnail for the image in the composition text field, when creating a thumbnail for a decrypted received MMS image that shows in the conversation, and most importantly, when resizing the image to 640x480 to send out.