Fix picture size truncation

When the target size of a picture conversion is computed, the values (width or height) are truncated, instead of rounded => it will cause an inconsistency between the metadata saved on the document and the real size of the generated picture.

For example,

1. Take a picture whose size is 1638x2000px
2. ImagingComponent#getSize will compute a size of 81x100px for the thumbnail
3. The search ratio would give 81,9x100 which should be rounded to 82x100
4. The conversion will call the command

   convert -define registry:temporary-path=/tmp -quiet -background white -flatten -depth 8 originalPix.jpeg[0] jpg:- | convert - -resize 81x100 thumbnail.jpg 

5. It produces a picture whose size is 81x99 (!= 81x100)