Hotfix V0.5.1 published

Changelog:
-Improved and optimized heightmap smoothing tool
-Fixed props, prop curves, roads and checkpoints not being set to newly generated terrain
-Fixed Cup Car's radiation breaking when hitting kerbs
-Fixed checkpoints being invisible in track builder
-Fixed database tracks not showing up in track builder
-Fixed inability to back out from load track -menu in track builder
-Fixed all ground matrials driving like tarmac in track builder test drive
-Fixed inability to zoom in and out in track builder after test drive
-Fixed some old tracks not loading
-Fixed other cars in multiplayer occasionally leaving gravel tire marks on tarmac
-Fixed first lap not counting after reset in multiplayer practice and qualifying
-Updated Cup car's base setup with a more stable one by Kinichie