We need to check on realize if we have access to a GL context, before
calling GL functions. We use gtk_gl_area_get_error() for that.
We also need to tear down the resources during unrealization, instead
of leaking them.
Make sure that variables are declared at the top of the block.
Break up one of the sincos() calls into individual calls to sin() and cos()
so that we do not have to complicate the initialization of the following
GLfloat array.
Instead of using glxgears, which still uses OpenGL 2.1 and the fixed
pipeline, we use a slightly modified es2gears, OpenGL 3.2, and the
programmable pipeline.
Its not really reasonable to handle failures to make_current, it
basically only happens if you pass invalid arguments to it, and
thats not something we trap on similar things on the X drawing side.
If GL is not supported that should be handled by the context creation
failing, and anything going wrong after that is essentially a critical
(or an async X error).
