ISSUE-6072 Fix image index overrun and add test for js convolution filter

[rfinck]

Previously, the convolution filter tried to extend beyond the boundaries of the image, which would attempt to add undefined to the last row and column of pixels and thus throw off the results.

This is the js half of #6072 ; the gl part is split out since it includes bigger changes to the testing setup.

[asturur]

this is good as it is.
I wonder if i should add a test suite of filters running on images, and compare results.