moved Emscripten-SDL changes from patch to a library
include alpha library functions
Put back the alpha-handling code

With the additional library, it's possible to work anyway.

This reverts commit 650611ed7de1d2de3fc497cbec2563e936bee82a.
Now the stub is in place, we can leave music alone

This reverts commit eed7fee18ee399e5f353b9cbba6b590c36d1969c.
improve build to allow native and emscripten side-by-side
rebuild html when deps change