[BUG] bad handling of u8 data in saveImage

[syurkevi]

saveImage should have explicit asserts for non-floating point data types. Saving u8 data through saveImage will result in corrupt image output from freeimage due to preprocessing steps. Either u8 data type needs to be preprocessed correctly or have explicit type checks to dissallow u8. Documentation needs to be updated to reflect allowable permutations of types and ranges, add differentiation between saveImage and saveImageNative()