Refactor pixel-mapping code for four-scan screens

[j-g00da]

Refactored the pixel-mapping code to use switch instead of nested ifs and improved indentation so it's easier to understand and add new PANEL_SCAN_RATEs in the future.

[j-g00da]

This is untested, please test if you have any four-scan screen.
@mrcodetastic @board707

[mrcodetastic]

Thanks for this. I will take a look over the coming days.

[board707]

@j-g00da
Thanks for your efforts.

[j-g00da]

Btw. I have 40px-high screens support and settable pixel-base ready, but these depend on this refactor, so I will open further PRs when this is merged.
Then we can create presets for common displays - let me know what do you think about this idea.

[board707]

Then we can create presets for common displays

in fact, there are no "common displays"... there are dozens of varieties of 4xscan matrices, and they all occur with approximately equal frequency.
So, to be honest, I don't see much benefit in creating presets in this part of the code. It would be more correct, as it seems to me, to give the user the ability to easily constructs a pixel mapping for his matrices, which is what I am trying to do in PR #739