merged branch fabpot/performance-fix (PR #941)

This PR was merged into the master branch.

Commits
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2450f79 changed the way globals behave to be more BC with 1.11 and to avoid speed problems at runtime introduced in 4487387 (closes #932)

Discussion
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changed the way globals behave to be more BC with 1.11 and to avoid speed problems at runtime introduced in 4487387 (closes #932)

* Globals are now managed by themselves, independently of other elements
  contained in extensions

* A global variable value can now be changed after the runtime has been
  initialized (to be more BC with the way Twig 1.11 works)

* Extensions are not initialized anymore when rendering a template that
  is already in the cache (like in Twig 1.11)