Context state overlap when initializing/releasing multiple contexts

Initializing a context after another one has been released may cause two contexts to share the same state, leading to crashes after one of the remaining contexts is also released. I've observed the following sequence of events:
```
initialize(0x1)
initialize(0x2)
s_bindings = {0x1 => 0, 0x2 => 1}
s_maxPos = 1

releaseContext(0x1)
s_bindings = {0x2 => 1}

initialize(0x3)
pos = s_bindings.size() --> 1, expected 2 (s_maxPos() + 1) or 0 (reusing the previously released slot)
s_bindings = {0x2 => 1, 0x3 => 1} --> BUG: both contexts share the same state!

releaseContext(0x2)
s_maxPos = 0 --> BUG: context 0x3's state is released!
s_bindings = {0x3 => 1}

later when using context 0x3:
assertion failed: AbstractFunction::maxPos() >= pos
```
(glbinding v3.1.0 from vcpkg - [relevant code](https://github.com/cginternals/glbinding/blob/6ae6961e653147670b8a724864fb44a960979b91/source/glbinding/source/Binding.cpp#LL186C43-L186C62) appears unchanged in v3.3)