Introduce QSGMaterial::CustomCompileStep

Nobody reimplements the virtual QSGMaterialShader::compile()
function, so we disable that code path and make it opt-in for
compatibility.

We do this, because opens up for that the renderer can do something
different, including changing the source code to allow for better
optimization.

Change-Id: Ia72f9eda7d6428e26878158ab03349f753a35c66
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>