Sun

- Implemented SSObjectBillboard for drawing camera-facing billboard objects
- It can optionally use occlusion queuery (and does when drawing the sun)
- Added  SSTextureWithAlpha for loading textures with alpha from SSAssetManager
- Impleneted SSObjectSunFlare that draws sun flare sprites in screen space
- Made scene render setup more consistent everywhere. 
- Removed code setting cull modes from object render functions. Setting cull mode is exclusive to the code managing multiuple scene renders
- All triangles everywhere are drawn in CCW (back-face culling)
- SSObjectMeshSky removed. Everything it does is already done by SSObjectMesh