Fully load all shaders to prevent future performance hiccups.
Usually graphics drivers do not actually prepare shaders until they are first needed. However when some object is first rendered with a previously unused shader, there can be a hiccup because the driver is compiling/optimizing the shader. This can be quite noticeable on mobile platforms.
Calling this function will perform dummy one-invisible-triangle rendering with all permutations of all currently loaded shaders. This can take some time but helps to avoid hiccups in the future.