feat: use fade-out effect on the filtered regions in the matrix

[sehilyi]

This uses the consistent 'fade-out' effect on the filtered-out items in the context view.

To draw a semi-transparent overlay, I basically separated this fade-out region into multiple rectangles. The following demonstrates how the white regions are actually drawn (stroke colors of individual rectangles being red):

Because we are not able to use data transforms (since Gosling uses HiGlass to render matrix, and HiGlass does not support filtering), I was not able to find an easier approach to implement the 'fade-out effect.' If data transforms are available, we could just, for example, overlay (1) a regular matrix, (2) a big semi-transparent white rectangle that covers the entire region of the matrix, and (3) a filtered matrix that shows only the selected samples.

[sehilyi]

Some screenshots after making a further change: