[3.15.5] Skyboxes and GoldSrc Mdl v6/v4 support
Main highlights of this release:
- Skyboxes are now imported along with Source1 maps
- GoldSrc models of versions 6 and 4 now can be imported (Half-Life Alpha build models)
- BVLG update to 3.4
- Added water shader and improved unlitgeneric shaders
Minor stability improvements in keyvalue parser.