Fixed typo in header comment.
Changed C++ style comments.
Align pointer in SDL_memset before doing Uint32 loop

Some more recent compilers emit SSE aligned store instructions for the loop,
causing crashes if the destination buffer isn't aligned on a 32-bit boundary.
This would also crash on platforms like ARM that require aligned stores.

This fixes a crash inside SDL_FillRect that happens with the official x64 mingw
build.

--HG--
extra : rebase_source : afd0f856fd814e7345bd0c7d20ddac3ba0422813