Expose InternalFormat in GammaCorrection

[ablot]

The GammaCorrection node expects 8-bit image as inputs. This can be an issue and make the post-gamma luminance of the screen step-like. Exposing the InternalFormat would solve the issue.
Maybe adding a small hint to say which pixel type are the most likely to be usefull in the description would be nice if you expose the property.

Use case examples where we cannot use the whole range and would want to use more than 256 values:
1- We have a projector with very bad dark. The first 50 gray values have exactly the same luminance. To do gamma correction we stretch the 205 other values, which means some steps
2- When using multiple monitors, we normalise the brightness of all screens by cropping the top and bottom of the curve (so that they have the same absolute luminance).