Prepares the blender for blending

obj.prepare(corners, sizes)
obj.prepare(dst_roi)

Input

• corners Source images top-left corners. Cell-array of 2D points {[x,y],...}.
• sizes Source image sizes. Cell-array of sizes {[w,h],...}.
• dst_roi Destination ROI rectangle [x,y,w,h], same as obtained by calling cv.Blender.resultRoi(corners,sizes).