Improve Channel Splitter painting

### Description

Check if we should also maybe add the channel separator size in `SamplesPainter::paint` `for loop` if we have more than one channel to draw, the idea is to ensure pixel perfect drawings,

At the moment the channel splitter is more than a pixel wide but defined as 1 pixel height so there might be some anti-aliasing happening, we could be drawing the channel splitter in C++ instead of QML to change that.