Small tweak to scrolling code
Make PixelAttributes more generic

In preparation for other changes
Change --tileorigin to accept ',' as separator

The previous separator (':') still works, but is not advertised.
Make the geometry pixel-accurate instead of map-block accurate

When requesting, for instance, a 75x85 map, the mapper will
now create a 75x85 map, instead of an 80x96 (or even 96x108)
map as it did before.

This new behavior is the default when using one of the options
--centergeometry or --cornergeometry.

In addition, both of these options will no longer shrink the
map, to remove rows or columns of empty blocks at the edges.
Previously, this behavior was enabled with --forcegeometry.

An option --geometrymode has been added as well, to tune
the interpretation of the geometry. It supports 4 flags:

- pixel:  the requested geometry is interpreted with pixel
	  granularity. The map is not enlarged to include
	  entire map blocks.

- block:  the requested geometry is interpreted with block
	  granularity. The map is enlarged with at most 15
	  nodes at each of the four edges, so that it
	  includes entire map blocks only.

- fixed:  a map of the requested geometry is created (after
	  adjustmens for 'block' mode). Empty rows or
	  columns at the edges are not removed.

- shrink: Empty rows and columns at the map edges are
	  removed to generate the smallest picture possible.

Lastly, a new geometry syntax has been added, which is more
compatible with known syntax (i.e. X-Windows), and which
allows the offset to be optional. If the offset is omitted,
the picture defaults to be centered around 0,0.

`<width>x<height>[+|-<xoffset>+|-<yoffset>]`

For compatibility, the behavior of the option --geometry
was not changed. If (and only if) used before --geometrymode,
it enables block granularity and shrink.

The old option --forcegeometry is no longer documented,
but still recognised for compatibility.
Improve layout of verbose geometry messages & tune their verbosity
Revert "Fix compiling with clang"

This problem was caused by my incorrect configuration.
Search more locations for the colors.txt file

The colors.txt file can now be located:
- in the directory of the world being mapped
- in the user's private minetest directory ($HOME/.minetest)
- in the current directory as a last resort

The location can also be specified using a command-line option
Abstraction of tile parameters computation to its own function.
Add CPack files to .gitignore