Automatic bidirectional alignment in x and y direction

[ralfgerlich]

This addresses bidirectional alignment as mentioned in #400 and y-alignment as mentioned in #486. These are the aspects implemented:

• Automatic horizontal (x-) texture alignment in both directions (Other minor auto-align quibbles #486)
• Automatic vertical (y-) texture alignment using vanilla DOOM single y-offset per side and taking pegging into account (Auto-align on the Y axis #400)

Aspects not implemented:

• auto-justification as mentioned in Other minor auto-align quibbles #486.
• application of individual y-offsets for upper, middle and lower wall part as supported by UDMF, for example.

This supersedes my previous pull request #1780 as I restructured the alignment implementation to minimize code duplication and extensive parameter passing, and extended the scope to generalized auto-alignment.

As this is limited to the single y-offset from vanilla DOOM, vertical alignment cannot be applied so that the whole wall is properly aligned, but instead only to part of a wall. The wall part selected when activating the alignment hotkey is taken as an indicator of user intent, i.e.

• If the upper part of a wall is selected, the intent is taken to be aligning upper parts of two-sided walls to each other.
• If the lower part of a wall is selected, the intent is taken to be aligning lower parts of two-sided walls to each other.
• If the midde part of a wall is selected, the intent is taken to be aligning middle parts of walls to each other.

When a one-sided wall is encountered while aligning with "upper wall" or "lower wall" intent, the middle section of the one-sided wall is aligned appropriately. Thus, taking the intent into consideration, this should mostly "just work as expected" (tm).

However, a wall is taken into consideration for alignment if any of its upper, middle or lower texture match the texture of the initial wall segment selected. If, for example, a one-sided wall is selected as initial wall by the user, alignment applies to the middle texture of two-sided walls, even if only the upper or lower wall texture matches that of the initial wall. In that case the non-matching middle texture of the two-sided wall would be aligned to the single middle texture of the one-sided wall.

This could be "fixed" by being more strict about the inclusion of walls into the alignment, but I have shied away from that as it would be a significant change away from the old behavior. However, if that is desired, I can amend this pull request appropriately.