Add resource manager lock, and ContextPtr to automatically take it

Affects: Everything, dEQP-EGL.functional.sharing.gles2.multithread.*
Bug: b/112184433
Change-Id: Ifdc5b18c738f92bbab08217f672a8ed6093e1672
Reviewed-on: https://swiftshader-review.googlesource.com/20388Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>