Support atlasing UI textures for Lua API

MyGUI supports atlasing, so implementation is fairly trivial, but we need to decide on the API.

I believe there are these key goals:

1. it should be possible to atlas UI that's initially designed with loose textures, without editing the UI layouts themselves. This will be particularly helpful with atlasing vanilla UI
2. it should be possible to define UI with atlas syntax, but use loose files in development, to make it easy to switch to an actual atlas later

Proposed API snippet:

local ui = require('openmw.ui')
local v2 = require('openmw.util').vector2

local myAtlas = ui.atlas{
  path = 'path/to/atlas.dds',
  tiles = {
    borderLeft = { -- name of the tile, used to refer to it later
      offset = v2(10, 15), -- position of the top left corner in the atlas
      size = v2(50, 50), -- size of the tile in pixels

      -- used only during script development, is expected to be replaced when the atlas is finalized
      path = 'fallback/non/atlased/texture.dds', -- use this texture instead of part of the atlas
    },
    -- ...
  },
}

local atlasedImageLayout = {
  type = ui.TYPE.Image,
  props = {
    atlasTile = myAtlas.borderLeft,
  },
}

-- some "legacy" layout definition, potentially in another script / mod
local normalImageLayout = {
  type = ui.TYPE.Image,
  props = {
    path = 'old/border/texture.dds',
  },
}

-- replaces all uses of 'old/border/texture.dds' path with the atlas tile, globally
ui.atlasOverride('old/border/texture.dds', myAtlas.borderLeft)