Compile shaders into the library.

This was long overdue; I was reluctant for a while since it's easier
to develop or debug shaders this way, but the solution is of course
to give priority to the filesystem.

It is now allowed to give "" as the data directory, which will only
ever look in the bundle. Most users should probably just do that.

Shaders are no longer installed by make install.

There is no API or ABI break, but of course, you cannot give ""
as directory to older versions (it would mean essentially /).