New formula: glfw3 (3.0.2)

This bumps gflw3 to 3.0.2.

Similarly to, say, libxml2, gtk2 and gtk3, this is separate from glfw (2.x), because:

- glfw3 is consistent with (most of) upstream's naming (libglfw3.a and libglfw3.h). This formula also builds glfw3.dylib, making it consistent either way. Dynamic build is now default.
- glfw3 and glfw 2.x will still need to be installed side by side: there is pyglfw and golang glfw which depend on 2.7, while go-gl/glfw3 needs glfw3.
- glfw 2.x supports Snow Leopard. From #20626:

    > Simply put GLFW 3.0.1 does not support Snow Leopard

PS: I did not put this in continuation of #21620 because it was not a proper branch. Also, in that previous, closed request I did not make it apparent how important it is for the two to be separate. I believe it is important to have both versions available.