Add ability to force assertions

[csciguy8]

This adds the ability for cesium-native asserts to execute in configurations where they are usually turned off.

A good example is with cesium-unreal. Unreal Engine lets their plugins run in Debug or Release, but the engine is built in Release (with NDEBUG present). Assertions get compiled out either way. In this situation, we can use the new CESIUM_FORCE_ASSERTIONS define with our debug build, to see debug assertions like we expect. This addresses cesium-unreal #1366.

To use, the native app needs to build cesium-native with the CESIUM_FORCE_ASSERTIONS preprocessor present. If defining this for a configuration that would normally have assertions anyway, it is ignored.

A side benefit is that we can now force assertions in Release builds. This could be useful in some situations, like when trying to diagnose bugs that happen in Release only.

[csciguy8]

This is a good place to start a review

[kring]

This looks good to me @csciguy8. The only small reservation I have is the name ASSERT. Macros can't be namespaced, so we're basically claiming this name globally. There's significant risk of a conflict. So even though it's more verbose, we should should probably namespace the name itself a bit. CESIUM_ASSERT, perhaps?

There's also a slightly different approach used by another project internal to Cesium that might be worth considering. I'll send you a link to that separately.