change the way 4K resolution is determined.

Older versions of
tvheadend incorrectly report 1080i channels as having 1088 vertical
pixels so they got classified as 4K. The algorithm is now changed
to consider source as 4K only when the total pixel count exceeds
6 megapixels.
Merge pull request #3680 from Jalle19/fix-4k-detection

fix false 4K detection on some 1080i and 3D sources