Portal-based visibility culling for WMO rendering (disabled by default,
needs debugging for complex WMOs like Stormwind). Skip character creation
screen when characters already exist in single-player mode.
- Add loading screen system with stb_image for JPEG loading
- Two loading screen images (orc and dwarf) randomly selected
- Display loading screen while terrain data loads
- Cache WMO inverse matrices to reduce per-frame computation
- Stub WMO liquid rendering (needs coordinate system fix)
- Update spawn point to Stormwind Trade District
The MOHD header structure was missing the nTextures field at the start,
causing all subsequent field reads to be offset by 4 bytes. This corrupted
bounding box values and caused floor geometry to be incorrectly culled.
