Rework graphical train HUD code

- A basic texture manipulation API is added; currently this is only a
  (selected) subset of texture modifiers provided by MT; the goal is to
  avoid writing (potentially incorrect) texture strings by hand;
- The graphical HUD code is cleaned up; in particular, most code used
  for generating texture patterns are moved to texture.lua so that the
  code can be used outside of the HUD;
- Inactive elements are given the darkslategray background.

A basic unittest is added; however, it needs to be expanded for better
coverage.

Reported-by: Lars Müller <appgurulars@gmx.de>

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