Fix OpenAL loading of extension function that are not present.

[NogginBops]

Purpose of this PR

Fixes an issue where checking if an OpenAL extension is present while the device and context doesn't have that extension present throws an error because it tries to load all of the extensions methods and fails.

Calling the extensions functions without it being present now gives a helpful error message.

Testing status

Manually tested checking for a made up non-existent AL extension which didn't crash.

Comments

This is not great for AOT style use cases as it generates IL at runtime. (we don't have any AOT use cases atm)
But this could be put behind if checks or static ifs if needed.

[frederikja163]

Not really sure if i understand whats going on here. But it looks good to go

[NogginBops]

This is doing the same type of dynamic code generation as found in #1179 . Adding that reference for completeness.

[NogginBops]

Merging.