Tweak ContextPtr to handle move construction/assignment correctly.
This was overlooked when I made the last minute change to use explicit lock/unlock.

Tweak LockGuard to allow construction from ptr to MutexLock, and behave
correctly if mutex == nullptr. This is required because the locking
machinery for EGL is /outside/ the error generation for bad EGLDisplay.

Add 'Big EGL Lock' around most of the EGL API

Bug: b/112184433
Change-Id: Ic4cd5fa9e6a16ae81cbb2b46e1cb881a850749c5
Reviewed-on: https://swiftshader-review.googlesource.com/20628Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>