prepare_gl

Checks that OpenGL is available for this and ensures that it is properly initialized.

When this fails, an throws will be set describing the error and this function returns false.

Note that even if this function succeeds, creating a `GdkGLContext` may still fail.

This function is idempotent. Calling it multiple times will just return the same value or error.

You never need to call this function, GDK will call it automatically as needed. But you can use it as a check when setting up code that might make use of OpenGL.

Parameters:

this	a `GdkDisplay`

Returns:

true if the display supports OpenGL