_getConvexHullForDrawable likely off by 1 pixel for non-integer sizes

[mzgoddard]

Expected Behavior

For the hull to match the underlying alpha component of the input image for both integer and non-integer sized images.

Actual Behavior

Silhouette and other ways to fetch the alpha information to build a Drawable's Hull needs to truncate or round up a skin's non-integer size so they can store a fixed number of pixels. If we fetch the info from those structures with a non-integer size vector, Silhouette automatically casts into its fixed size space. Some amount of the pixels fetched to build the hull will likely be shifted for non-integer sized skins.

[mzgoddard]

Silhouette doesn't seem to be truncating the width and height to and this will cause non-integer skins to always return true from Silhouette.isTouching as the index used to fetch the info for a cell is also non-integer. That lookup will return undefined instead of a Uint8 value and undefined !== 0 will make the call always true if the vector is in the bounds of the Silhouette.