Generate shaders to cover all possible cases.

With this change, all the vertex and fragment shaders used by the GL
renderer are now generated based on a program description supplied
by the caller. This allows the renderer to generate a large number
of shaders without having to write all the possible combinations by
hand. The generated shaders are stored in a program cache.

Change-Id: If54d286e77ae021c724d42090da476df12a18ebb