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.