Add xcb dynamic loading support

[mwestphal]

I agree that my contributions are licensed under the MIT License.
You can find a copy of this license at https://opensource.org/licenses/MIT

This adds supports for loading xcb dynamically instead of linking against it, by providing all needed symbols as function pointers.

TODO: Error management

Fix: #98
Fix: f3d-app/f3d#3326

[mwestphal]

Error management is not done, I did not find a way to convey an clean error if the lib or a symbol is not available. There is an error code stuff but I'd need to be able to abort right during the manager creation.
In that case I usually would use an exception and document it in the API doc, but there is not a single exception in your API, please let me know.

@dacap

[mwestphal]

@Meakk please take a look

@dacap please review

[Meakk]

a few comments

[Meakk]

Looks good to me but I think @dacap wanted to have a CMake variable to switch between link and dynamic load mode.
Let's see what they think about dropping the link mode.

[mwestphal]

Looks good to me but I think @dacap wanted to have a CMake variable to switch between link and dynamic load mode. Let's see what they think about dropping the link mode.

Right, completely forgot about that.

[mwestphal]

@dacap in the meantime I have added a simple exception system

[dacap]

I'll take a look at this later, but I don't want to include a third party license with clip (as it will obligate to include that license to everybody who is using clip). I'd implement the loading of xcb functions if it's required (the code is not as complex to be copied from other impl).

[mwestphal]

I don't want to include a third party license with clip

Ha you mean the fact that I copied from VTK ?
As you see fit, you can just keep it in the header.
I can also rewrite the copied part if that helps, know that I understand how to do it ^^

Let me know.