Fix for incorrect subpixel precision of marker when using track offset

Issue was caused by the way how pattern sampling happens in case of
anchored display: track offset is applying on search buffer which
means offset is rounding to an integer. Fractional pat of offset was
completely ignoring which lead to jumps in pattern buffer.

This was only a visualization issue in track preview widget.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55028 954f8c5b-7b00-dc11-b283-0030488c597c