Fixed documentation errors and typos:
 - Wrong required temporary memory was size given in the API comments
   and readme.
 - Minor typo fix.
Fixed using wrong stride value in sdfCoverageToDistanceField().
Fix to https://github.com/memononen/SDF/issues/2
Example program improvements:
 - A new test image added with text and some problematic cases
 - Use 'A' to toggle alpha testing on/off (to see the distance field)
 - Added panning the image with arrow keys
 - Use pgUp/pgDown to zoom
 - Use home/end to change SDF radius
A better fix for https://github.com/memononen/SDF/issues/2
Merge pull request #3 from suikki/master

SDF bug fixes