Revert "Bug 942 - Share GLBufferStateTracker ; Unify GLBufferStateTracker and GLBufferSizeTracker (simplification)"

This reverts commit 7c5483d5b20aed9c87c5ce3f6bc840b6546edcd1.
Bug 942: Bug 942 - Review GLBuffer[State|Size]Tracker and NIO mapped buffers

Commit f8a74c9831c65725a699320c27e62161a0378241 reverted
commit 7c5483d5b20aed9c87c5ce3f6bc840b6546edcd1
due to the fact that the buffer binding itself is _not_
shared across shared GLContext!

Apply uncritical changes of 7c5483d5b20aed9c87c5ce3f6bc840b6546edcd1:

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Simplify GLBufferSizeTracker creation @ GLContextImpl ctor,
make it final.

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Clear the GLBufferSizeTracker (@destruction) only if no more
created shares are left!

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Refine API doc.

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