Scanline fixes

[ryanfelder]

This resolves two issues with scanline rendering :

• Burn-in does not respect/include rasterization (scanlines or pixels)
• Scanlines end abruptly

Example of burn-in issue:

Example of both issues fixed:

Example of "scanlines end abruptly":

Similar screenshot with horizontal-only scanline blurring enabled (5%):

I've never used QML before, and I know nothing about shaders, so apologies for the brute-force solutions.