Move extension handling from WGL to DXVA2 interop

Depends on !813 (merged) because of the wgl changes.

The only users of those extensions are interop modules, so only interop_dxva2 for extension related to WGL integration.

By moving the loading of those extension to the interop, we can have them working even when the OpenGL platform context is provided by the vglmem (vgl.c) module callbacks, allowing hardware acceleration even when using the libvlc OpenGL callbacks.

Filters would likely be using only OpenGL extensions that are already available, but could be using the same mechanism in the worst-case scenario.

Refs #25234, #25256 (closed)