Add vsync support to the software renderer

[cooljeanius]

Previously I had requested that the Wesnoth project use vsync in this downstream bug: wesnoth/wesnoth#5018
However, upon investigation, devs discovered that the reason why that's not possible is because apparently it's actually an upstream issue with SDL 2 here, in that the software renderer doesn't support vsync, which is apparently due to this line:

SDL/src/render/software/SDL_render_sw.c

Line 875 in 391bb80

SDL_RENDERER_SOFTWARE | SDL_RENDERER_TARGETTEXTURE,

It would be nice if something in this area could be fixed here on SDL's end.

[cooljeanius]

oh wow, thanks for the quick fix!

[slouken]

You're welcome! :)