Upgrade bundled version of SDL to 2.0.9

[rasky]

Please upgrade the bundled version of SDL to 2.0.9, which includes a fix for MacOS Mojave where canvas is not presented until the window is moved/resized once.

[mattiascibien]

I second this. By providing 2.0.9 (or at least 2.0.8) it will be possibile to have Vulkan with bundled.

[rombankzero]

The bundled SDL version was reverted from 2.0.8 to 2.0.5 in 0472c00, though I have no idea why. It breaks other things too (see #816).

[mattiascibien]

@rombankzero I think it was due to a bug in 2.0.6 (and upper I think) #781 (comment). However I think it should have been fixed in 2.0.9 https://hg.libsdl.org/SDL/rev/c0b17b32b95e and the changes for 2.0.9 include that commit https://hg.libsdl.org/SDL/file/8feb5da6f2fb/CMakeLists.txt

[rombankzero]

@mattiascibien Ah, I figured there had to be a reason!

Bumping those version numbers in build.rs to 2.0.9 would be nice. I guess in principle the in-tree SDL headers and bindings should be updated too, though I'm not sure what the usual procedure is there. I'm having no luck with bindgen on Windows; it's just spewing massive, inscrutable error messages at me...