`GlfwWsiDriver::getInstanceExtensions` was creating an `std::vector` with a size argument in the ctor but then used `push_back` instead of filling the pre-allocated elements, leading to a bunch of nullptr entries at the start that caused an exception later on when accessed.
Since we're not linking to the libraries anymore, it doesn't make much sense to
use find_library, and in fact we need to use dependency() in order to get the
right CFLAGS for includes, defines, etc, so use that instead.
As a result, we can remove the 'SDL2/' folders from the includes, making the SDL
includes more correct.
Removing these link-time dependencies is important for making a single binary that is compatible with either backend, regardless of whether or not each one is currently available to the program.
Rather than directly calling functions, the API now calls shared functions that call into a WsiDriver instance, which is allocated and implemented by the backend. Functionally this should be the same, it just has the extra allocation for the function table.
This prepares the WSI library for supporting multiple implementations in a single binary.
