Add height map support.
Improve voxel shadow rendering.
Force at least one render per tick during regular gameplay.

Partially reverts #5850, as this feature is a serious usability regression when controlling units. Fixes #6867.
Account for terrain height in Viewport.
Add terrain geometry visualisation.
Add a new test map.
Move NullLoadScreen, DefaultLoadScreen and LoadWidgetAtGameStart to Mods.Common
Move some Warheads to Mods.Common
Merge pull request #6901 from pchote/heightmaps-part-two

Implement terrain heightmaps
Merge pull request #6906 from reaperrr/common-10

Moved LoadScreens and some Warheads to Mods.Common
Merge pull request #6900 from pchote/fix-gameplay-skipping

Force at least one render per tick during regular gameplay.
Update make version to check for git.

This will prevent the version from being set to null if git isn't in the
path.
Update make to not have issues with pausing at the end.
Merge pull request #6816 from steelphase-forks/bleed-ps-make-version

Update make version to check for git.
Added IDisabledTrait & rewrote upgrade code using a level-based approach.

	Upgradeable traits are notified whenever an upgrade of their declared types are granted or revoked.  The traits maintain their own internal level counter, which is then used to enable or disable the trait functionality.  A trait can register for multiple upgrade types which then all affect the internal level counter.

	IDisabledTrait for identifying (and filtering) disabled traits
	UpgradableTrait provides an abstract base for traits to support upgrade levels
	Added IDisabledTrait support to GlobalButtonOrderGenerator

	Includes rework by pchote with alterations.