With this commit, custom backends can now be implemented as
self-contained modules that can easily be plugged in and mixed and
matched depending on a programs requirements. The order in which
custom backends are specified in the context config determine their
priority.
This commit updates the custom_backend example by moving the SDL
code out into its own file in "extras/backends/sdl". The example will
now just include the SDL code files like normal. This represents a more
realistic scenario.
David Reid