Implement Order-Independent Transparency

[fu5ha]

Implements weighted-blended order independent transparency.

• Lowest GL required is now 4.1. This is still several years old and shouldn't be a problem: It's supported back to late 2011/2012 Macs and probably a similar time for other computers.
• Now draws into intermediate framebuffers and then blits into the one provided by Qt at the end
• Now uses regular OpenGL to draw the final render instead of using QPainter

[oktomus]

There is only one thing stopping your code from building on linux :)