[feat] - viewer / batch convert - Change color depth to 8bit + Filter = mess

[user0]

XnView MP 1.4.0 64bits
Windows 64bit (22H2), display resolution 3840x2160, win scaling 150%

Viewer (same behaviour in Batch convert)
Change color depth to 8bit + Filter = mess

To reproduce:
1. run clean portable XnViewMP
2. open any .jpg (e.g. 24bit, no alpha, no metadata)
3. open 'Change color depth' dialog
- set Colors 256 (any dithering)
- press OK
- (OPTIONAL) save as .png, close xnview and open this .png
4. open 'Filter' dialog
- tick 'Apply to image' - for easy check
- click on each filter
most of the Filters show mess

change_color_depth_and_filters.jpg

Test file in 'Batch convert'

test_png.zip

batch_convert_mess.jpg

[xnview]

yes normal. The only way is to convert the image into 24bits