Fix casting error when converting from int8 to uint16.
Fix casting error when converting from int8 to uint16.

This error only comes up in development versions of numpy, which refuses to cast the input to the correct type for inplace operations.

bennlich/scikit-image,emmanuelle/scikits.image,michaelaye/scikit-image,oew1v07/scikit-image,vighneshbirodkar/scikit-image,youprofit/scikit-image,emmanuelle/scikits.image,SamHames/scikit-image,Midafi/scikit-image,michaelpacer/scikit-image,chintak/scikit-image,GaZ3ll3/scikit-image,robintw/scikit-image,WarrenWeckesser/scikits-image,chintak/scikit-image,paalge/scikit-image,ClinicalGraphics/scikit-image,keflavich/scikit-image,ClinicalGraphics/scikit-image,jwiggins/scikit-image,bsipocz/scikit-image,Midafi/scikit-image,SamHames/scikit-image,almarklein/scikit-image,warmspringwinds/scikit-image,michaelpacer/scikit-image,almarklein/scikit-image,robintw/scikit-image,chintak/scikit-image,almarklein/scikit-image,keflavich/scikit-image,ofgulban/scikit-image,chriscrosscutler/scikit-image,Hiyorimi/scikit-image,newville/scikit-image,dpshelio/scikit-image,ajaybhat/scikit-image,bennlich/scikit-image,blink1073/scikit-image,ajaybhat/scikit-image,emmanuelle/scikits.image,bsipocz/scikit-image,blink1073/scikit-image,emon10005/scikit-image,rjeli/scikit-image,juliusbierk/scikit-image,chintak/scikit-image,rjeli/scikit-image,emmanuelle/scikits.image,youprofit/scikit-image,Britefury/scikit-image,jwiggins/scikit-image,michaelaye/scikit-image,emon10005/scikit-image,juliusbierk/scikit-image,almarklein/scikit-image,SamHames/scikit-image,chriscrosscutler/scikit-image,paalge/scikit-image,WarrenWeckesser/scikits-image,newville/scikit-image,GaZ3ll3/scikit-image,Britefury/scikit-image,paalge/scikit-image,dpshelio/scikit-image,oew1v07/scikit-image,warmspringwinds/scikit-image,pratapvardhan/scikit-image,rjeli/scikit-image,pratapvardhan/scikit-image,SamHames/scikit-image,vighneshbirodkar/scikit-image,vighneshbirodkar/scikit-image,ofgulban/scikit-image,Hiyorimi/scikit-image,ofgulban/scikit-image