Added error handler (agl_error_handler and agl_set_error_handler).
Added agl_atomic_compr_and_swap_ptr.
Added agl_assert.
Added $(BACKEND) to makefiles and AGL_BACKEND to defines.
Defined AGL_ERROR_REQUEST.
Added AGL_EOUTOFMEMORY and agl::Error::OutOfMemory.
Moved agl_resource_create and agl_resource_destroy into agl.private.h.
Resource creation is now done per-type, e.g., agl_query_create.
Updated methods to use agl_assert.
Added agl_command_list_t and agl::CommandList.
Added agl_resource_queue_for_create, and agl_resource_queue_for_destroy.
Added agl_command_length_from_type.