d3d12, gallium: Misc patches for D3D12 GLES and external texture support

The first few patches here just enable some trivial formats/caps that should've been enabled from the start. Adding support to the DXIL compiler for external textures is also trivial, since they're identical to 2D - any complex stuff gets lowered away by mesa/st.

Then there's a few patches to enable:

• Importing external textures on Windows, where shared handles are HANDLE (aka void*), not int fds.
• The D3D12 backend specifically importing external textures. The WDDM way of sharing resources embeds all of the type information in the resource handle, rather than requiring it to be passed separately along with the handle, so we introspect the handle and make sure it matches the externally-provided resource definition. If it doesn't, we can't use it, so we reject it.