Split tileset artwork loading out of TileSet.

This simplifies terrain loading and allows for
non-square tiles in game.

The editor still relies on the old code for now.
Introduce TileSetRenderer for editor and utility.
Use TileSetRenderer for utility --tmp-png.
Use TileSetRenderer in the editor.
Remove TileSet rendering code.
Use Manifest.TileSize everywhere.
Allow tilesets to specify their sheet size.

Decreased the default size from 4096 to 512.
Remove obsolete IRenderAsTerrain.

This removes the technical limitation preventing
FrozenUnderFog on bridges.
Support .r8 tileset artwork.
Use uppercase extensions to support case-sensitive filesystems.
Merge pull request #3684 from pchote/tileset-fixup

TileSet Refactoring
load the Dune 2000 mouse curser from the native MOUSE.R8 format
load Dune 2000 walls directly from DATA.R8
moved the towers to DATA.R8 sequences
migrate all other buildings to DATA.R8 sequences

4th conyard might be for the Corrino faction
TODO: active animations are yet unused overlays

no need to render build-top for refineries
replace the infantry sequences with transposed DATA.R8 frames
load Dune 2000 aircraft directly from DATA.R8
load projectiles, muzzles and explosions from DATA.R8
move vehicles to load sprites from DATA.R8 directly
read the production icons from sequences

this allows d2k to read them from DATA.R8 without trouble