introduce a findall version of mapwindow

[johnnychen94]

Originally posted in #223 (review)

We have mapwindow that applies function f on every local patch of an image, which can be seen as the image version of map. We don't have the window version of findall that applies f to each local patch, and return coordinates such that f(window_of_p) == true.

Why we need this? We can currently do this by returning a dense Bool matrix from mapwindow and then call findall, but this would usually be unnecessary and not as performant as the manually written for-loops. For example, the benchmark in JuliaImages/ImageSegmentation.jl#72 (comment)