Previous commits broke it... :(

Backface culling is enabled by default for all tiles, as this
is how the lua parser initializes each tiledef. We revert to
always using the value from the tiledef since it is always
read and serialized.

Mods that wish to enable culling for e.g. mesh nodes, now can
specify the following to enable backface culling:

    tiles = {{ name = "tex.png", backface_culling = true }},

Note the double '{' and use of 'name' key here! In the same
fashion, backface_culling can be disabled for any node now.

I've tested this against the new door models and this properly
allows me to disable culling per node. I've also tested this
against my crops mod which uses mesh nodes where culling needs
to be disabled, and tested also with plantlike drawtype nodes
where we want this to continue to be disabled.

No default setting has changed. The defaults are just migrated
from nodedef.cpp to c_content.cpp.

corrected this bit reflect the function properly.

The save button is now fully functional again when an error message
is shown.

After an invalid value is entered in the settings tab dialog, the GUI
label for the error message that is shown was partly overlapping the
'save' button, so that the top half of the button could not be clicked
on.

fixes minetest/minetest_game#786

cloneMesh() has to use a switch in order to create a different
mesh buffer type depending on vertex type. (Credit: the new cloneMesh
was written by RealBadAngel.)

To avoid repetitive code, all other methods use getVertexPitchFromType()
to automatically adapt the indexing to the vertex type at runtime.

Lua_api.txt: Document 'minetest.registered_biomes'
Minimal: Remove 'mapgen_air' alias
Cavegen: Add fallback node for 'mapgen_ice'
Dungeongen: Add fallback node for 'mapgen_river_water_source'
Mgv5: Remove unnecessary '#include util/directiontables.h'
Add missing 'this->'s in makeChunk()
Mgv6: Edit empty line formatting
Remove leading spaces in makeChunk()
Add missing spaces after 'for' and 'if'
Mgv7: Edit empty line formatting

Changes:
- Accept setting an empty flags-type value in the settings tab
  if the variable specification permits it
- Don't accept substrings of flag values
  E.g. with values: 'one,two,three', 'hree', 'w', etc. used to
  be accepted. Not any more
- Don't accept flags with random pattern-matching special characters
  E.g. with values: 'one,two,three', 'on.', '(o)[n]e*' etc. used
  to be accepted. Not any more.

- Accept numbers prefixed with '+'
- Accept multiple spaces instead of just a single one where spaces are expected
- Allow flags to have an empty default value

I apparently forgot to add this file in my previous commit (bd40ee2b).

The pageflip mode requires a stereo quadbuffer, and a modern graphic
card. Patch tested with NVidia 3D Vision.

The mini-map is not drawn, but that's what is done for topbottom and
sidebyside modes as well.

Also most of the time the user would prefer the HUD to be off. That's
for the user to decide though, and toggle it manually.

Finally, the interocular distance (aka eye separation) is twice as much
as the "3d_paralax_strength" settings. I find this a strange design
decision. I didn't want to chance this though, since it's how the other
3d modes interpret this settings.

Fixed spelling mistake!

set_sky does work with on_joinplayer

Previously, we assumed that reply->str was NULL
terminated. However, this turned out to be not true,
as users reported crashes in strlen connected to
where reply->str was appended to an std::string.

Use the method recomended by the docs, to read the
length separately.

Add new node property 'floodable', default false
Define "air" as floodable = true in C++ and lua

* Add main page.
* Organize into sections.
* Add threading sources.
* Include SpatialAreaStore, LevelDB/Redis, sound, FreeType, and cURL in output.
* Add logo.
* Fix project name hardcoding.
* Remove PAPER_TYPE (only used when GENERATE_LATEX is enabled).
* Have dot render graphs as SVG (smaller, and works even if dot's text drawing functionality is broken).
* Enable built-in STL support.
* Enable search bar.
* Switch from header-bar based navigation to treeview based navigation.
* Enable dynamic HTML (collapses graphs).
* Enable generation timestamp.

Fix for commit 87dcee6a It uses the wrong variable and only covers some use cases. This change covers all use cases.

Removing the target file on other platforms was enabled likely unintentionally
by commit 5f1f1151.

This may be the reason why there has been corruption of files on Linux on hard
shutdowns.

Previously I described the problem and this fix in issue #3084.

My commit

e2d54c9f "shutdown when requested from lua in singleplayer too"

broke minetest's feature to connect to servers. The client crashed
after the connection init was complete.

Thanks to @sofar for reporting the bug.

Fixes #3498.

If an `XDG_CACHE_HOME` can't be found or `RUN_IN_PLACE` is enabled,
`path_cache` is left at its default of `$PATH_USER/cache`
(at a time when `PATH_USER` is `..`), rather than being reset to
`$PATH_USER/cache` after `PATH_USER` has been properly set.