Port SDL 1.2 code to SDL 2.0

[planetmaker]

Also, as I've noted in the pull request (https://github.com/OpenTTD/OpenTTD/pull/7086), I'm specifying the SDL 2 renderer as a software renderer for stability reasons. Messing with the GPU sounds nice but introduces some problems that may not work on everyone's setups.

We do some funky stuff with the processor mmx extensionsions.... maybe similar can be done. But that's definitely out-of-scope for a PR which just establishes SDL2 - more a follow-up.

[njn]

Alright, well my PR is now out of "work-in-progress" state. Please take a look and see if you can break things with regard to changing resolutions, or any other problems. And also, we'll need to make sure keyboard shortcuts and input are all working correctly: SDL2 handles text input in a different way than 1.2, so there were some significant changes in that area required in this video driver.