Use #version 120 explicitly, and adapt the shader shader code
accordingly, to use strictly only GLSL 1.20 constructs. Also, use less
vertex attribute data in the Vortex vertex shader: We can pack the
per-vertex tile x and y index and in-tile vertex index information
into one float. Also, the shader can calculate the center of the tile
a vertex belongs to based on the knowledge of which tile it is.

Now the shader transitions work on OS X, too.

Change-Id: I93e8b5069a6d06d2e412ffee322b1eb32805e606