Consider zoom when culling billboards.

screenScale represents the current zoom factor, but may need some
revision because I'm not sure if what it's doing is technically correct.
It is used to multiply the culling distance for cells, as well as the
fading distance.
Added comment.
Fix engine API copy/paste errors (typo).
Update squish library to 1.11
Removed hard coded directory paths

- Removed references to core directory.
- Switch some references to tools directory
- Added preferences variables where appropriate
Merge pull request #248 from DavidWyand-GG/HardCodedPaths2

Removed hard coded directory paths
Updating libpng to v1.5.14 and zlib to v1.2.7

Since libpng has a dependency upon zlib, both updates were combined for one commit/pull-request.
Merge pull request #250 from thecelloman/updatelibpngandzlib

Updating libpng to v1.5.14 and zlib to v1.2.7
Merge pull request #243 from thecelloman/updatesquish

Update squish
Update TinyXML to v2.6.2
Merge pull request #255 from thecelloman/updatetinyxml

Update TinyXML to v2.6.2
Tools GuiProfile Separation.

This effectively duplicates (where necessary) and moves tools specific GuiProfiles out of the core and into the tools.  This will allow future modifications to either the game guis or tools guis that use these profiles to not conflict with each other.
Remove unused and (partially) unworking UnifiedShell.
Merge pull request #259 from thecelloman/toolsguiseparation

Tools GuiProfile Separation.
Merge pull request #260 from thecelloman/removeUshell

Remove unused and (partially) unworking UnifiedShell.
Relocates several game specific GUIs (with relevant art and scripts) out of the "core".
Merge pull request #264 from thecelloman/gameguisarenotcore

Relocate game specific GUIs  out of the "core"
Removing the controlsHelpDlg to reduce maintenance overhead.
Merge pull request #266 from thecelloman/removecontrolshelpdlg

Removing the controlsHelpDlg to reduce maintenance overhead.
Made the flipped y-axis on terrain import optional.

Pass a final boolean argument to TerrainBlock::import to control
y-axis flipping. It is enabled by default, since this was the previous
default behavior.

This should be added as an option in the terrain import dialog - see
game/tools/worldEditor/gui/guiTerrainImportGui.gui