libobs/graphics: Add support for NT Handle shared textures

[VodBox]

Description

Adds support for using shared textures that were made with the
D3D11_RESOURCE_MISC_SHARED_NTHANDLE flag.

This increases the minimum required Windows version to Windows 8 or the
Platform Update for Windows 7. As official support is only for Win 8+
this does not change official support.

Motivation and Context

The new shared texture method required by our CEF branch uses shared textures made with the D3D11_RESOURCE_MISC_SHARED_NTHANDLE and D3D11_RESOURCE_MISC_SHARED_KEYEDMUTEX flags, so we need support for NT Handles.

How Has This Been Tested?

This has been tested against an implementation of the new shared texture method used in our CEF branch.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[notr1ch]

Should this also be split into separate commits? It affects both libobs-d3d11 and libobs.