Fully load shaders in ShaderVariantCollection.
Often 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, especially on mobile platforms.
Calling this function will perform dummy one-invisible-triangle rendering
for the shaders and their variants in this ShaderVariantCollection. You can setup to do that
automatically during game load time (in Graphics Settings), or manually call this function at
appropriate times (e.g. when loading a new level).
Calling WarmUp on a ShaderVariantCollection that is already warmed up will do nothing.