Update SDL 1.2.15 to Revision 8040

Fix for SDL_GetGammaRamp and SDL_SetGammaRamp crash on MacOSX 10.9.x
Also possibly fixes Bug 6093: https://bugzilla.icculus.org/show_bug.cgi?id=6093
Various other platform/bug fixes since 1.2.15 was released in January of 2012