Specifications developed in PR #3206 require that groundcover content files must not be allowed to corrupt normal content files. With this PR we simply isolate our existing loading logic by instantiating a separate `ESMStore` for `Groundcover`. In addition, we remove some outdated workarounds.
This PR aims to solve all issues with `Groundcover` view distance handling in a satisfying way while preserving other optimisations that benefit other features. The main idea here is not to rely on `ViewData` updates for distance culling calculations because we can not accurately determine distance against lazily updated views. Instead, we perform an accurate measurement in a cull callback we can run every frame in `Groundcover` itself. While we do have to add some code to handle this feature, it is quite loosely coupled code that could be useful elsewhere in the future. These changes should address a performance regression @akortunov experienced.
This PR aims to solve `uniform block LightBufferBinding has no binding` messages @glassmancody has reportedly encountered since PR #3110 due to an apparent bug in OSG. While we do have to add a workaround here that adds a bit of clunkiness, #3216 should allow us to clean up these interactions a bit in the future.
With this PR we test out osg's shader define system for a somewhat harmless feature. As we can see, our code becomes more concise and efficient in this case. Most importantly, we no longer create unneeded vertex shader objects.
