drawTexture edge condition enforcement behavior is not well defined for all subimage cases

Currently drawTexture enforces edge conditions on the outermost 1/2 pixel of a subimage. This makes sense when thinking of subimages that are an integer number of pixels, but since we allow subimage coordinates that are arbitrary floating point values we might end up with the case that it is called on a subimage area that is less than 1 pixel across and so the edge conditions will not only completely take over the operation, but the area that is half a pixel in from the sides will be a negative size.