Custom Bloom Threshold Value

**Is your feature request related to a problem? Please describe.**
Currently, _threshold_ is just a boolean in the PostProcessing options (Options.h) - however, internally the material uses a float for thresholding anyways. However, currently there is no way to change the threshold for Bloom, which can be an issue when attempting to use the effect in a controllable manner.

**Describe the solution you'd like**
Replacing or adding an option to the bloom settings to set a custom threshold value.

**Describe alternatives you've considered**
Hardcoding a custom bloom threshold, but that's... more of a hack and not really necessary.

**OS and backend**
Not specific.