VK: Add external texture.

[bkaradzic]

No description provided.

[bkaradzic]

cc @dbartolini

[dbartolini]

Thanks for all your hard work. I guess next you'll add a platform API to obtain external shared texture details?

[bkaradzic]

Yes, I'm still thinking how this API should look like. But currently you could add your VK changes to create shared texture. Basically on creation you use BGFX_TEXTURE_EXTERNAL_SHARED and _external should be 0. That would create shared texture. You should use your API to obtain shared handle for now.