Fix parallel build

Use $(builddir) for built objects in the current build
directory, instead of $(top_builddir)/src. The 2nd version
breaks dependency tracking.
Fix build when not building with gtk-gl support
Move tests dir to top-level
Build gtk-doc by default when running autogen.sh
Don't access non-built headers in $(builddir)
Add a simple README about the module
Fix docs build when building without GtkGL support