Reload assets when source file changes

[mickvangelderen]

Automatically reload textures, models, shaders, heightmaps, etc. when their source file changes to speed up development. Having to restart the application and get to the right state every time is not fantastic. Will have to keep some sort of dependency graph somehow. If you reload a shader you will have to relink dependent programs and requery uniform locations for example.

It has to work really well, otherwise you have to sometimes restart the application and will end up doing it always "just to be sure".