decide which mirror to choose locally and display result

closes #3904
tweak: artillery min. range slight increase

Artillery was recently buffed, but I think this makes the fact that they can currently fire at close range is not appropriate. I think artillery should be vulnerable to units that are able to get quite close, instead of being able to fire at them and do massive damage.

The reason min. range of 2c896 was picked is that when you set min. range to 3c0, it is not able to fire exactly 3 cells away. Perhaps the behaviour should change so that 3c0 = 3 cells away. 

(For comparison, MLRS has minimum range of 3).
port Tao.OpenAl to OpenTK.Audio.OpenAL
StyleCop
Fixed removal of winforms for x86
Reduce RangeLimit of Rockets, BikeRockets

To give some units a chance to escape, limiting RangeLimit of these rockets. Basically it is because if you get within range with a vehicle, but then quickly turn around to get out of range, the missile will follow you for quite a long ways, well beyond its initial range. There is currently no way to escape the missile. This allows a unit which is very fast to stand a chance of escaping a shot instead of having no chance.
Nerf Chem warrior vs. Wood, 50% -> 35%

Since Chem warriors are awesome vs. everything, I think it would make sense to make them not quite as awesome vs. buildings. There is a bit of a lack of dinstinction between chem and flamers, and right now it doesn't appear to players like it makes much sense to build flamers when you can build chems instead. This will keep "burning buildings down" as the forte of the flame throwers. 
The chems will still be decent vs. wood, but not as crazy as they have been.
Increase ScanRadius of Commando & Rocket Soldiers

Making ScanRadius value equal their range (6).
Updated infantry unit descriptions

There were a few misconceptions.
Updated structure descriptions

Provided more explanation, eliminated ambiguity.

Where necessary, I wrote "cannot target aircraft" instead of "weak vs. aircraft", since the latter is ambiguous.
Updated vehicle descriptions. Changed AA desc.

"Weak vs. Aircraft" is kind of ambiguous, so I replaced it. Instead, I specified which units can attack aircraft by giving them a separate line reading "Can attack Aircraft". All entries of "weak vs. aircraft" have been removed.
port Tao.OpenGl to OpenTK.Graphics.OpenGL and avoid Arb

closes #4540
only query for the extension we need
give a hint when someone forgot to install drivers on Windows
check for OpenGL version 2.0+
Merge pull request #5256 from Unit158/winforms-patch-2

Fixed removal of winforms for x86
Merge pull request #4843 from Mailaender/local-mirrors

Moved mirror selection to the client
OpenRA.Thirdparty is now OpenRA.Support
Merge pull request #5273 from Mailaender/support-random-bitrot

OpenRA.Thirdparty is now OpenRA.Support
Merge pull request #5248 from Mailaender/sdl2-opentk

Ported Tao.OpenAL/OpenGL to SDL2/OpenTK